[234]
As the Mexican–American War brought the debate over slavery in the territories to the forefront of American politics, Van Buren published an anti-slavery manifesto. In it, he refuted the notion that Congress did not have the power to regulate slavery in the territories, and argued the Founding Fathers had favored the eventual abolition of slavery.[235] The document, which became known as the "Barnburner Manifesto," was edited at Van Buren's request by John Van Buren and Samuel Tilden, both of whom were leaders of the Barnburner faction.[236] After the publication of the Barnburner Manifesto, many Barnburners urged the former president to seek his old office in the 1848 presidential election.[237] The 1848 Democratic National Convention seated competing Barnburner and Hunker delegations from New York, but the Barnburners walked out of the convention when Lewis Cass, who opposed congressional regulation of slavery in the territories, was nominated on the fourth ballot.[238]
In response to the nomination of Cass, the Barnburners began to organize as a third party. At a convention held in June 1848, in Utica, New York, the Barnburners nominated Van Buren for president.