The danger of Van Buren's difference with Jackson it was sought to avert. Butler visited Jackson at the Hermitage, and doubtless showed him for what a sinister end he had been used. Jackson did not withdraw his approval of annexation; but publicly declared his regard for Van Buren to be so great, his confidence in Van Buren's love of country to be so strengthened by long intimacy, that no difference about Texas could change his opinions. Van Buren's nomination was again widely supposed to be assured. But the work of Calhoun and Robert J. Walker had been too well done. The convention met at Baltimore on May 27, 1844. George Bancroft headed the delegation from Massachusetts. Before the Rev. Dr. Johns had "fervently addressed the Throne of Grace" or the Rev. Mr. McJilton had "read a scripture lesson," the real contest took place over the adoption of the rule requiring a two thirds vote for a nomination. For it was through this rule that enough Southern members, chosen before Van Buren's letter as they had been, were to escape obedience to their instructions to vote for him. Robert J. Walker, then a senator from Mississippi, a man of interesting history and large ability, led the Southerners. He quoted the precedent of 1832, when Van Buren had been nominated for the vice-presidency under the two thirds rule, and that of 1835, when he had been nominated for the presidency. These nominations had led to victory. In 1840 the rule had not been adopted. Without this rule, he said amid angry excitement, the party would yield to those whose motto seemed to be "rule or ruin." Butler, Daniel S. Dickinson, and Marcus Morton led the Northern ranks. Butler regretted that any member should condescend to the allusion to 1840. That year, he said, had been a debauchery of the nation's reason amid log cabins, hard cider, and coon-skins; and in an ecstasy of painful excitement at the recollection and amid a tremendous burst of applause "he leaped from the floor and stamped ... as if treading beneath his feet the object of his loathing." The true Democratic rule, he continued, required the minority to submit to the majority. Morton said that under the majority rule Jefferson had been nominated; that rule had governed state, county, and township conventions. Butler admitted that under the rule Van Buren would not be nominated, although a majority of the convention was known to be for him. In 1832 and 1835 the two thirds rule had prevailed because it was certainly known who would be nominated; and the rule operated to aid not to defeat the majority. If the rule were adopted, it would be by the votes of States which were not Democratic, and would bring "dismemberment and final breaking up of the party." Walker laughed at Butler's "tall vaulting" from the floor; and, refusing to shrink from the Van Buren issue, he protested against New York dictation, and warningly said that, if Van Buren were nominated, Clay would be elected. After the convention had received with enthusiasm a floral gift from a Democratic lady whom the President declared to be fairer than the flowers, the vote was taken. The two thirds rule was adopted by 148 to 118. All the negatives were Northerners, except 14 from Missouri, Maryland, and North Carolina. Fifty-eight true "Northern men with Southern principles" joined ninety Southerners in the affirmative. It was really a vote on Van Buren,—or rather upon the annexation of Texas,—or rather still upon the extension of American slave territory. It was the first battle, a sort of Bull Run, in the last and great political campaign between the interests of slavery and those of freedom.

On the first ballot for the candidate, Van Buren had 146 votes, 13 more than a majority. If after the vote on the two thirds rule anything more were required to show that some of these votes were given in mere formal obedience to instructions, the second ballot brought the proof. Van Buren then sank to 127, less than a majority; and on the seventh ballot to 99. A motion was made to declare him the nominee as the choice of a majority of the convention; and there followed a scene of fury, the President bawling for order amid savage taunts between North and South, and bitter denunciations of the treachery of some of those who had pledged themselves for Van Buren. Samuel Young of New York declared the "abominable Texas question" to be the fire-brand thrown among them by the "mongrel administration at Washington," whose hero was now doubtless fiddling while Rome was burning. Nero seems to have been Calhoun, though between the god-like young devil of antiquity wreathed with sensual frenzy and infamy, and the solemn, even saturnine figure of the great modern advocate of human slavery, the likeness seemed rather slight. The motion was declared out of order; and the name of James K. Polk was presented as that of "a pure whole-hogged Democrat." On the eighth ballot he had 44 votes. Then followed the magnanimous scene of "union and harmony" which has so often, after a conflict, charmed a political body into unworthy surrender. The great delegation from New York retired during the ninth balloting; and returned to a convention profoundly silent but thrilling with that bastard sense of coming glory in which a lately tumultuous and quarreling body waits the solution of its difficulties already known to be reached but not yet declared. Butler quoted a letter which Van Buren had given him authorizing the withdrawal of his name if it were necessary for harmony; he eulogized Polk as a strict constructionist, and closed by reading a letter from Jackson fervently urging Van Buren's nomination. Daniel S. Dickinson said that "he loved this convention because it had acted so like the masses," and cast New York's 35 votes for Polk. The latter's nomination was declared with the utmost joy, and sent to Washington over Morse's first telegraph line, just completed. Silas Wright of New York, Van Buren's strong friend and a known opponent of annexation, was, in the fashion since followed, nominated for the vice-presidency, to soothe the feelings and the conscience of the defeated. Wright peremptorily telegraphed his refusal. He told his friends that he did "not choose to ride behind on the black pony." George M. Dallas of Pennsylvania took his place.

The Democratic party now threw away all advantage of the issue made by the undeserved defeat four years before. Thirty-six years later it repeated the blunder in discarding Van Buren's famous neighbor and disciple. Polk's was the first nomination by the party of a man of the second or of even a lower rank. Polk was known to have ability inferior not only to that of Van Buren and Calhoun, but to Cass, Buchanan, Wright, and others. He was the first presidential "dark horse," and indeed hardly that. His own State of Tennessee had, by resolution, presented him as its choice for vice-president with Van Buren in the first place. He had been speaker of the national House, and later, governor of his State; but since holding these places had been twice defeated for governor. In accepting the nomination he declared, with an apparent fling at Van Buren, that, if elected, he should not accept a renomination, and should thus enable the party in 1848 to make "a free selection."

The nomination aroused disgust enough. "Polk! Great God, what a nomination!" Letcher, the Whig governor of Kentucky, wrote to Buchanan. But the experiment of 1840 with the Whigs had been disastrous; the people had swung back to the strict doctrines of the Democracy. Van Buren faithfully kept his promise to support the nomination; under his urgency Wright finally accepted the nomination for governor of New York. And by the vote of New York Henry Clay was defeated by a man vastly his inferior. Polk had 5000 plurality in that State; but Wright had 10,000. Had not James G. Birney, the abolitionist candidate who polled there 15,812 votes, been in the field, not even Van Buren's party loyalty would have prevented Clay's election. Van Buren's friends saved the State; but in doing so voted for annexation. In April, 1844, Clay had written a letter against annexation. As it appeared within a few days of Van Buren's letter, and as the personal relations between the two great party leaders were most friendly, some have inferred an arrangement between them to take the question out of politics. This would indeed have been an extraordinary occurrence. One might well wish to have overheard a negotiation between two rivals for the presidency to exclude a great question distasteful to both. After the Democratic convention, Tyler's treaty of annexation was rejected in the Senate by 35 to 16, six Democrats from the North, among them Wright of New York and Benton of Missouri, voting against it. During the campaign Clay had weakly abandoned even the mild emphasis of his first opposition, and by flings at the abolitionists had openly bid for the pro-slavery vote; thus perhaps losing enough votes in New York to Birney to defeat him. After the election the current for annexation seemed too strong; and a resolution passed both Houses authorizing the admission of Texas as a State. The resolution provided for the formation of four additional States out of Texas. In any such additional State formed north of the Missouri compromise line, slavery was to be prohibited; but in those south of it slavery was to be permitted or prohibited as the inhabitants might choose.

Slavery was now clearly before the political conscience of the nation. Van Buren was the conspicuous victim of the first encounter. The Baltimore convention had in its platform complimented "their illustrious fellow-citizen," "his inflexible fidelity to the Constitution," his "ability, integrity, and firmness," and had tendered to him, "in honorable retirement," the assurance of the deeply-seated "confidence, affection, and respect of the American Democracy." This sentence to "honorable retirement" Van Buren, who was only in his sixty-second year and in the amplitude of his natural powers, received with outward complacency. On the eve of the election he pointed out, probably referring to Cass, that the hostility to him had not been in the interest of Polk, and warmly said that, unless the Democratic creed were a delusion, personal feelings ought to be turned to nothing. Van Buren was, however, profoundly affected by what he deemed the undeserved Southern hostility to himself. For he hardly yet appreciated that his defeat was politically legitimate, and not the result of political treachery or envy. Between him and the Southern politicians had opened a true and deep division over the greatest single question in American politics since Jefferson's election.

With Polk's accession and the Mexican war, the schism in the Democratic ranks over the extension of American slave territory became plainer. Even during the canvass of 1844 a circular had been issued by William Cullen Bryant, David Dudley Field, John W. Edmonds, and other Van Buren men, supporting Polk, but urging the choice of congressmen opposed to annexation. Early in the new administration the division of New York Democrats into "Barnburners" and "Old Hunkers" appeared. The former were the strong pro-Van Buren, anti-Texas men, or "radical Democrats," who were likened to the farmer who burned his barn to clear it of rats. The latter were the "Northern men with Southern principles," the supporters of annexation, and the respectable, dull men of easy consciences, who were said to hanker after the offices. The Barnburners were led by men of really eminent ability and exalted character: Silas Wright, then governor, Benjamin F. Butler, John A. Dix, chosen in 1845 to the United States Senate, Azariah C. Flagg, the famous comptroller, and John Van Buren, the ex-President's son, and a singularly picturesque figure in politics, who was, in 1845, made attorney-general by the legislature. He had been familiarly called "Prince John" since his travels abroad during his father's presidency. Daniel S. Dickinson and William L. Marcy were the chief figures in the Hunker ranks. Polk seemed inclined, at the beginning, to favor, or at least to placate, the Barnburners. He offered the Treasury to Wright, though he is said to have known that Wright could not leave the governorship. He offered Butler the War Department, but the latter's devotion to his profession, for which he had resigned the attorney-general's place in Van Buren's cabinet, made him prefer the freedom of the United States attorneyship at New York, and Marcy was finally given the New York place in the cabinet. Jackson's death in June, 1845, deprived the Van Buren men of the tremendous moral weight which his name carried, and which might have daunted Polk. It perhaps also helped to loosen the weight of party ties on the Van Buren men. After this the schism rapidly grew. In the fall election of 1845 the Barnburners pretty thoroughly controlled the Democratic party of the State in hostility to the Mexican war, which the annexation of Texas had now brought. Samuel J. Tilden of Columbia county, and a profound admirer of Van Buren, became one of their younger leaders.

Now arose the strife over the "Wilmot Proviso," in which was embodied the opposition to the extension of slavery into new Territories. Upon this proviso the modern Republican party was formed eight years later; upon it, fourteen years later, Abraham Lincoln was chosen president; and upon it began the war for the Union, out of whose throes came the vastly grander and unsought beneficence of complete emancipation. David Wilmot was a Democratic member of Congress from Pennsylvania; in New York he would have been a Barnburner. In 1846 a bill was pending to appropriate $3,000,000 for use by the President in a purchase of territory from Mexico as part of a peace. Wilmot proposed an amendment that slavery should be excluded from any territory so acquired. All the Democratic members, as well as the Whigs from New York, and most strongly the Van Buren or Wright men, supported the proviso. The Democratic legislature approved it by the votes of the Whigs with the Barnburners and the Soft Hunkers, the latter being Hunkers less friendly to slavery. It passed the House at Washington, but was rejected by the Senate, not so quickly open to popular sentiment. In the Democratic convention of New York, in October, 1846, the "war for the extension of slavery" was charged by the Barnburners on the Hunkers. The former were victorious, and Silas Wright was renominated for governor, to be defeated, however, at the election. Polk, Marcy, and Dickinson, angered at the Democratic opposition in New York to the pro-slavery Mexican policy, now threw all the weight of federal patronage against the Barnburners, many of whom believed the administration to have been responsible for Wright's defeat. Van Buren and his influence were completely separated from the national administration. Just before the adjournment of Congress in 1847, the appropriation to secure territory from Mexico was again proposed. Again the Wilmot Proviso was added in the House; again it was rejected in the Senate, to the defeat of the appropriation; and again Barnburners and Whigs carried in the New York legislature a resolution approving it, and directing the New York senators to support it.

The tide was rising. It seemed that Mexican law prohibited slavery in New Mexico and California, and that upon their cession the principles of international law would preserve their condition of freedom. Benton, therefore, deemed the Wilmot Proviso unnecessary; a "thing of nothing in itself, and seized upon to conflagrate the States and dissolve the Union." For the Supreme Court had not then pronounced slavery a necessary accompaniment of American supremacy. But the legal protection of freedom was practically unsubstantial, even if not technical; there could be no doubt of the determination of the South to carry slavery into these Territories, whatever might be the obligations of either municipal or international law; and their conquest, therefore, made imminent a decision of the vital question whether slavery should be still further extended.

At the Democratic convention at Syracuse, in September, 1847, the Hunkers, after a fierce struggle over contested seats, seized control of the body. David Dudley Field, for the Barnburners, proposed a resolution that, although the Democracy of New York would faithfully adhere to the compromises of the Constitution and maintain the reserved rights of the States, they would still declare, since the crisis had come, "their uncompromising hostility to the extension of slavery into territory now free." This was defeated. The Barnburners then seceded, and issued an address, in which Lawrence Van Buren, the ex-President's brother, joined. They protested that the anti-slavery resolution had been defeated by a fraudulent organization of the convention, and called a mass meeting at Herkimer, on October 26, "to avow their principles and consult as to future action." The Herkimer convention was really an important preliminary to the formation of the modern Republican party. It was a gathering of the ex-President's friends. Cambreleng, his old associate, presided; David Wilmot addressed the meeting; and John Van Buren, now very conspicuous in politics, reported the resolutions. In these the fraud at Syracuse was again denounced; a convention was called for Washington's birthday in 1848, to choose Barnburner delegates to contest the seats of those chosen by the Hunkers in the national Democratic convention. It was declared that the freemen of New York would not submit to slavery in the conquered provinces; and that, against the threat of Democrats at the South that they would support no candidate for the presidency who did not assent to the extension of slavery, the Democrats of New York would proclaim their determination to vote for no candidate who did so assent.