He Ran For Presidency Again In 1848 As A Third Party Candidate

Van Buren released an anti-slavery manifesto as the Mexican-American War brought the discussion about slavery in the territories to the fore of American politics. He stated that the Founding Fathers had supported the eventual eradication of slavery in it and disproved the claim that Congress lacked the authority to control slavery in the territories. John Van Buren and Samuel Tilden, two of the Barnburner faction's leaders, rewrote the paper that came to be known as the "Barnburner Manifesto” at Van Buren's request. Many Barnburners wanted the former president to run for reelection in the 1848 presidential election, following the release of the Barnburner Manifesto. Competing New York delegations from Barnburner and Hunker were seated at the 1848 Democratic National Convention, but the Barnburners left when Lewis Cass, who opposed legislative control of slavery in the territories, was selected on the fourth ballot.

The Barnburners started coming together as a third party in response to Cass' nomination. The Barnburners proposed Van Buren as their candidate for president at a meeting held in Utica, New York, in June 1848. Van Buren accepted the candidacy despite being hesitant to leave the Democratic Party in order to demonstrate the strength of the anti-slavery movement, support the defeat of Cass, and undermine the Hunkers. A collection of anti-slavery Democrats, Whigs, and members of the abolitionist Liberty Party gathered in Buffalo, New York in August 1848 for the inaugural national convention of what would later be known as the Free Soil Party.