It was clear that Van Buren sympathized with all this. Relieved from the constraint of power, there strongly revived his old hostility to slavery; he recalled his vote twenty-eight years before against admitting Missouri otherwise than free. He now perceived how profound had really been the political division between him and the Southern Democrats when, in 1844, he wrote his Texas letter. Ignoring the legitimate character of the politics of Polk's administration in denying official recognition or reward to Barnburners,—legitimate if, as Van Buren had himself pretty uniformly maintained, patronage should go to friends rather than enemies, and if, as was obvious, there had arisen a true political division upon principles,—Van Buren was now touched with anger at the proscription of his friends. Excluded from the power which ought to have belonged to the chief of Democrats enjoying even in "honorable retirement" the "confidence, affection, and respect" of his party, independence rapidly grew less heinous in his eyes. One can hardly doubt that there now more freely welled up in his mind, to clarify its vision, the sense of personal wrong which, since Polk's nomination, had been so long held in magnanimous and dignified restraint,—though of this he was probably unconscious. Van Buren was not insincere when, in October, 1847, he wrote from Lindenwald to an enthusiastic Democratic editor in Pennsylvania, who had hoisted his name to the top of his columns for 1848. Whatever, he said, had been his aspirations in the past, he now had no desire to be President; every day confirmed him in the political opinions to which he had adhered. Conscious of always having done his duty to the people to the best of his ability, he had "no heart burnings to be allayed and no resentments to be gratified by a restoration of power." Life at Lindenwald was entirely adapted to his taste; and he was (so he wrote, and so doubtless he had forced himself to think) "sincerely and heartily desirous to wear the honors and enjoyments of private life uninterruptedly to the end." If tendered a unanimous Democratic support with the assurance of the election it would bring, he should not "hesitate respectfully and gratefully, but decidedly to decline it," adding, however, the proviso so precious to public men, "consulting only my own feelings and wishes." It was in the last degree improbable, he said,—and so it was,—that any emergency should arise in which this indulgence of his own preferences would, in the opinion of his true and faithful friends, conflict with his duty to the party to which his whole life had been devoted, and to which he owed any personal sacrifice. The Mexican war had, he said, been so completely sanctioned by the government that it must be carried through; and, he ominously added, the propriety of thereafter instituting inquiries into the necessity of its occurrence, so as to fix the just responsibility to public opinion of public servants, was then out of season. Not a word of praise did he speak of Polk's administration; in this he was for once truly and grimly "non-committal."

In the New York canvass of 1847, the Barnburners, after their secession, "talked of indifferent matters." The Whigs were therefore completely successful. In the legislature the Barnburners, or "Free-soilers" as they began to be called, outnumbered the Hunkers. Dickinson proposed in the Senate at Washington a resolution, the precursor of Douglas's "squatter sovereignty,"—that all questions concerning the domestic policy of the Territories should be left to their legislatures to be chosen by their people. Lewis Cass, now the coming candidate of the South, asserted in December, 1847, the same proposition, pointing out that, if Congress could abolish the relation of master and servant in the Territories, it might in like manner treat the relation of husband and wife. After this "Nicholson letter" of his, Cass might well have been asked whether he would have approved the admission of a State where the last relation was forbidden, and where concubinage existed as a "domestic institution." Dickinson's proposal meant that the first settlers of each Territory should determine it to freedom or to slavery; it meant that in admitting new States the nation ought to be indifferent to their laws on slavery. If slavery were a mere incident in the polity of the State, a matter of taste or convenience, the proposition would have been true enough. But euphemistic talk about "domestic institutions" blinded none but theorists or lovers of slavery to the truth that slavery was a fearful and barbarous power, and that it must become paramount in any new Southern State, monstrous and corrupting in its tendencies towards savagery, unyielding, wasteful, and ruinous,—a power whose corruption and savagery, whose waste and ruin, debauched and enfeebled all communities closely allied to the States which maintained it,—a power in whose rapid growth, in whose affirmative and dictatorial arrogance, and in the intellectual ability and even the moral excellences of the aristocracy which administered it at the South, there was an appalling menace. As well might one propose the admission to political intimacy and national unity of a State whose laws encouraged leprosy or required the funeral oblations of the suttee. If there were already slave States in the confederacy, it was no less true that the nation had profoundly suffered from their slavery. Nor could all the phrases of constitutional lawyers make the slave-block, the black laws, and all the practices of this barbarism mere local peculiarities, distasteful perhaps to the North but not concerning it, peculiarities to be ranked with laws of descent or judicial procedure. Cass and Dickinson for their surrender to the South were now called "dough-faces" and "slavocrats" by the Democratic Free-soilers. They were the true "Northern men with Southern principles."

The Barnburners met at Utica on February 16, an earlier day than that first appointed, John Van Buren again being the chief figure. The convention praised John A. Dix for supporting the Wilmot Proviso; and declared that Benton, a senator from a slave State, but now a sturdy opponent of extending the evil, and long the warm friend and admirer of Van Buren, had "won a proud preëminence among the statesmen of the day." Delegates were chosen to the national convention to oppose the Hunkers. In April, 1848, the Barnburner members of the legislature issued an address, the authors of which were long afterwards disclosed by Samuel J. Tilden to be himself and Martin and John Van Buren. At great length it demonstrated the Free-soil principles of the Democratic fathers.

The national convention assembled in May, 1848. It offered to admit the Barnburner and Hunker delegations together to cast the vote of the State. The Barnburners rejected the compromise as a simple nullification of the vote of the State, and then withdrew. Lewis Cass was nominated for president, the Wilmot Proviso being thus emphatically condemned. For Cass had declared in favor of letting the new Territories themselves decide upon slavery. The Barnburners, returning to a great meeting in the City Hall Park at New York, cried, "The lash has resounded through the halls of the Capitol!" and condemned the cowardice of Northern senators who had voted with the South. Among the letters read was one from Franklin Pierce, who had in 1844 voted against annexation, a letter which years afterwards was, with a reference to his famous friend and biographer, called the "Scarlet Letter." The delegates issued an address written by Tilden, fearlessly calling Democrats to independent action. In June a Barnburner convention met at Utica. Its president, Samuel Young, who had refused at the convention at Baltimore in 1844 to vote for Polk when the rest of his delegation surrendered, said that if the convention did its duty, a clap of political thunder would in November "make the propagandists of slavery shake like Belshazzar." Butler, John Van Buren, and Preston King, afterwards a Republican senator, were there. David Dudley Field read an explicit declaration from the ex-President against the action and the candidates of the national convention. This letter, whose prolixity is an extreme illustration of Van Buren's literary fault, created a profound impression. He declared his "unchangeable determination never again to be a candidate for public office." The requirement by the national convention that the New York delegates should pledge themselves to vote for any candidate who might be nominated was, he said, an indignity of the rankest character. The Virginia delegates had been permitted, without incurring a threat of exclusion, to declare that they would not support a certain nominee. The convention had not allowed the Democrats of New York fair representation, and its acts did not therefore bind them.

The point of political regularity, when discussed upon a technical basis, was, however, by no means clear. The real question was whether the surrender of the power of Congress over the Territories, and the refusal to use that power to exclude slavery, accorded with Democratic principles. On this Van Buren was most explicit. Jefferson had proposed freedom for the Northwest Territories; and all the representatives from the slaveholding States had voted for the ordinance. Not only Washington and the elder and younger Adams had signed bills imposing freedom as the condition of admitting new Territories or States, but those undoubted Democrats, Jefferson, Madison, Monroe, and Jackson, had signed such bills; and so had he himself in 1838 in the case of Iowa. This power of Congress was part of "the compromises of the Constitution," compromises which, "deeply penetrated" as he had been "by the convictions that slavery was the only subject that could endanger our blessed Union," he had, he was aware, gone further to sustain against Northern attacks than many of his best friends approved. He would go no further. As the national convention had rejected this old doctrine of the Democracy, he should not vote for its candidate, General Cass; and if there were no other candidate but General Taylor, he should not vote for president. If our ancestors, when the opinion and conduct of the world about slavery were very different, had rescued from slavery the territory now making five great States, should we, he asked, in these later days, after the gigantic efforts of Great Britain for freedom, and when nearly all mankind were convinced of its evils, doom to slavery a territory from which as many more new States might be made. He counseled moderation and forbearance, but still a firm resistance to injustice.

This powerful declaration from the old chief of the Democracy was decisive with the convention. Van Buren was nominated for president, and Henry Dodge, a Democratic senator of Wisconsin, for vice-president. Dodge, however, declined, proud though he would be, as he said, to have his name under other circumstances associated with Van Buren's. But his State had been represented in the Baltimore convention; and as one of its citizens he cordially concurred in the nomination of Cass. A national convention was called to meet at Buffalo on August 9, 1848.

Charles Francis Adams, the son of John Quincy Adams, presided at the Buffalo convention; and in it Joshua R. Giddings, the famous abolitionist, and Salmon P. Chase were conspicuous. To the unspeakable horror of every Hunker there participated in the deliberations a negro, the Rev. Mr. Ward. Butler reported the resolutions in words whose inspiration is still fresh and ringing. They were assembled, it was said, "to secure free soil for a free people;" the Democratic and Whig organizations had been dissolved, the one by stifling the voice of a great constituency, the other by abandoning its principles for mere availability. Remembering the example of their fathers in the first declaration of independence, they now, putting their trust in God, planted themselves on the national platform of freedom in opposition to the sectional platform of slavery; they proposed no interference with slavery in any State, but its prohibition in the Territories then free; for Congress, they said, had "no more power to make a slave than to make a king." There must be no more compromises with slavery. They accepted the issue forced upon them by the slave power; and to its demand for more slave States and more slave Territories, their calm and final answer was, "no more slave States and no more slave territory." At the close were the stirring and memorable words: "We inscribe on our banner, Free Soil, Free Speech, Free Labor, and Free Men; and under it we will fight on and fight ever, until a triumphant victory shall reward our exertions."

Joshua Leavitt of Massachusetts, one of the "blackest" of abolitionists, reported to the convention the name of Martin Van Buren for president. After the convention was over, even Gerrit Smith, the ultra-abolitionist candidate, declared that, of all the candidates whom there was the least reason to believe the convention would nominate, Van Buren was his preference. The nomination was enthusiastically made by acclamation, after Van Buren had on an informal ballot received 159 votes to 129 cast for John P. Hale. A brief letter from Van Buren was read, declaring that his nomination at Utica had been against his earnest wishes; that he had yielded because his obligation to the friends, who had now gone so far, required him to abide by their decision that his name was necessary to enable "the ever faithful Democracy of New York to sustain themselves in the extraordinary position into which they have been driven by the injustice of others;" but that the abandonment at Buffalo of his Utica nomination would be most satisfactory to his feelings and wishes. The exclusion of slavery from the Territories was an object, he said, "sacred in the sight of heaven, the accomplishment of which is due to the memories of the great and just men long since, we trust, made perfect in its courts." Charles Francis Adams was nominated for vice-president; and dazzled and incredulous eyes beheld on a presidential ticket with Martin Van Buren the son of one of his oldest and bitterest adversaries. That adversary had died a few months before, the best of his honors being his latest, those won in a querulous but valiant old age, in a fiery fight for freedom.

In September, John A. Dix, then a Democratic senator, accepted the Free-soil nomination for governor of New York. The Democratic party was aghast. The schismatics had suddenly gained great dignity and importance. Martin Van Buren, the venerable leader of the party, its most famous and distinguished member, this courtly, cautious statesman,—could it be he rushing from that "honorable retirement," to whose safe retreat his party had committed him with so deep an affection, to consort with long-haired and wild-eyed abolitionists! He was the arch "apostate," leading fiends of disunion who would rather rule in hell than serve in heaven. Where now was his boasted loyalty to the party? Rage struggled with loathing. All the ancient stories told of him by Whig enemies were revived, and believed by those who had long treated them with contempt. It is clear, however, that Van Buren's attitude was in no wise inconsistent with his record. His party had never pronounced for the extension of slavery; nor had he. The Buffalo convention was silent upon abolition in the District of Columbia. There was for the time in politics but one question, and that was born of the annexation of Texas,—Shall slavery go into free territory? As amid the clash of arms the laws are stilled, so in the great fight for human freedom, the independent treasury, the tariff, and internal improvements could no longer divide Americans.

The Whigs had in June nominated Taylor, one of the two heroes of the Mexican war. It is a curious fact that Taylor had been authoritatively sounded by the Free-soil leaders as to an acceptance of their nomination. Clay and Webster were now discarded by their party for this bluff soldier, a Louisiana slaveholder of unknown politics; and with entire propriety and perfect caution the Whigs made no platform. A declaration against the extension of slavery was voted down. Webster said at Marshfield, after indignation at Taylor's nomination had a little worn away, that for "the leader of the Free-spoil party" to "become the leader of the Free-soil party would be a joke to shake his sides and mine." The anti-slavery Whigs hesitated for a time; but Seward of New York and Horace Greeley in the New York "Tribune" finally led most of them to Taylor rather than, as Seward said, engage in "guerrilla warfare" under Van Buren. Whigs must not, he added, leave the ranks because of the Whig affront to Clay and Webster. "Is it not," he finely, though for the occasion sophistically, said, "by popular injustice that greatness is burnished?" This launching of the modern Republican party was, strangely enough, to include in New York few besides Democrats. In November, 1847, the Liberty or Abolition party nominated John P. Hale for president; but upon Van Buren's nomination he was withdrawn.