In 1843, the officers of the First Regiment of Artillery elected Mr. Wilson its Major without his knowledge. He accepted the position, and in June, 1846, he was chosen Colonel, and was elected Brigadier General of the Third Brigade in August, which position he continued to hold for five years.
In March, 1848, a Whig district convention was held at Dedham, to nominate a candidate for Congress to fill the vacancy occasioned by the death of John Quincy Adams. Henry Wilson, Horace Mann, and William Jackson, were the leading candidates. After three ballotings Mr. Wilson declined being considered a candidate, and Mr. Mann was nominated. The convention, at the same time, by an almost unanimous vote, elected Mr. Wilson a delegate to the National Whig Convention. That the vote was not unanimous was owing to the fact that he had stated in public and in private that if General Taylor should be fixed upon by the Whig party as its candidate, unpledged to the Wilmot Proviso, he not only would not support him, but would do all in his power to defeat him.
When General Taylor was nominated, and the Wilmot Proviso voted down by the Whig National Convention, in June, 1848, General Wilson, and his colleague, Hon. Charles Allen, denounced the action of the convention, and left it. Gen. Wilson then got up a meeting of a few northern men, which was held in the evening, to consider what steps should be taken.
Gen. Wilson called the meeting to order, and after stating its purposes, moved the appointment of a committee to call a convention of the opponents of the Slave Power. The committee was accordingly appointed, and united with others in calling the Buffalo Convention, which nominated Mr. Van Buren and Mr. Chas. Francis Adams.
In the summer of 1848, General Wilson purchased the "Boston Republican" a free-soil newspaper, which he edited from January, 1849, to January, 1851, during which two years he gave his whole time to the free-soil cause, and spent more than seven thousand dollars of his own property, in the support of the newspaper, whose continued existence was deemed essential to the welfare of the party of which it was the organ. In 1849, he was chosen chairman of the Free-soil State Committee, in which capacity he acted for four years. In the fall of 1849, a coalition was formed between the free-soilers, and the Democrats of Middlesex County, for the election of senators, and General Wilson was pressed by both parties to stand as a candidate for the Senate, which he steadfastly refused to do. He was, however, in that year, chosen a representative from the town of Natick. When the legislature met, he was unanimously nominated by the free-soilers, as their candidate for Speaker. During the session, he was in his seat every day, always attentive to business.
After Mr. Webster made his seventh of March speech, an effort was made to instruct him to vote for the doctrines embodied in the resolutions pending before the legislature; but the proposition was resisted, and voted down by the Whig majority. General Wilson told the House that the people would repudiate that speech, and the men who indorsed it, and that at the coming election, the men who had deserted the cause of freedom would be crushed by the people. This prediction, which was received with defiance by the Whig leaders, was fulfilled, and no one in Massachusetts contributed more to its fulfillment than the man who made it.
In the summer of 1850, General Wilson called together, at the Adams House in Boston, the State Committee, and the leading men of the free-soil party, to the number of about seventy. He stated to the meeting that the people would make a coalition; that it would be successful if the committee would aid it; that Mr. Webster's seventh of March speech could be rebuked; the Fillmore administration condemned; a free-soiler sent to the United States Senate in place of Mr. Webster for the long term; and an anti-compromise Democrat for the short term; and in short, that by a coalition, Massachusetts could be placed in such a position that the anti-slavery men could control her policy. After a debate of five hours, in which Messrs. Marcus Morton, Samuel Hoar, J. G. Palfrey, C. F. Adams, R. H. Dana, Jr., and others took part, the meeting declined to sanction the coalition, only nine gentlemen, and they the youngest present, advocating the coalition. The people, however, made it, in spite of the disapprobation of the eminent men, and the State was carried against the Whigs, and Geo. S. Boutwell made Governor, and Charles Sumner and Robert Rantoul sent to the United States Senate.
In 1850, General Wilson was unanimously nominated for senator from Middlesex County by the free-soil and Democratic conventions, and elected by twenty-one hundred majority. When the legislature met, he was chosen President of the Senate. In 1851, he was reëlected and again chosen president. While President of the Senate, he was made Chairman of the Committee to welcome President Fillmore to Massachusetts, and also Chairman of the Committee to welcome Kossuth.
In 1852, he was a delegate to the free-soil National Convention at Pittsburg, and was selected to preside over that body, and also made Chairman of the National Free-soil Committee. In the same year, he was unanimously nominated for Congress by the free-soilers of the eighth district, and, although the majority against the free-soilers in that district exceeded seventy-five hundred, he failed of an election by only ninety-three votes. A large portion of the free-soilers desired him that year to be a candidate for Governor, and most of the coalition Democrats likewise desired his nomination. In a public letter he peremptorily declined to be a candidate, notwithstanding which he received more than a third of the votes of the Free-soil State Convention at Lowell.
In March, 1853, General Wilson was elected to the Constitutional Convention by the town of Berlin, and also by his own town of Natick. He was not absent from the convention for an hour during the session, and the journal and report of the debates show the active part taken by him in its transactions. During the temporary illness of the president, Mr. Banks, he was chosen president pro tem. In September, 1853, he was nominated by the Free Democratic State Convention, as candidate for the office of Governor. Out of six hundred votes cast by the convention, he received all but three. At the time he was nominated, men of all parties conceded the probability of his election. But the letter of Caleb Cushing, denouncing, in behalf of the administration, the coöperation of Democrats and free-soilers in State affairs—the bitter hostility of conservatism toward the new constitution, and the Irish vote against it—all contributed to overthrow the State reform party, and to defeat General Wilson and his friends.