[239] Though reluctant to bolt from the Democratic Party, Van Buren accepted the nomination to show the power of the anti-slavery movement, help defeat Cass, and weaken the Hunkers.[240] At a convention held in Buffalo, New York in August 1848, a group of anti-slavery Democrats, Whigs, and members of the abolitionist Liberty Party met in the first national convention of what became known as the Free Soil Party.[241]
The convention unanimously nominated Van Buren, and chose Charles Francis Adams as Van Buren's running mate. In a public message accepting the nomination, Van Buren gave his full support for the Wilmot Proviso, a proposed law that would ban slavery in all territories acquired from Mexico in the Mexican–American War.[241] Anti-slavery Whig orator Daniel Webster, in his "Marshfield Speech," expressed skepticism, in terms that may have influenced Whig voters, about the sincerity of Van Buren's espousal of the anti-slavery cause:

.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 40px}.mw-parser-output .templatequote .templatequotecite{line-height:1.5em;text-align:left;padding-left:1.6em;margin-top:0}In Mr. Van Buren's anti-slavery professions, Mr. Webster had no confidence. He said pleasantly, but significantly, that "if he and Mr. Van Buren should meet under the Free-soil flag, the latter with his accustomed good-nature would laugh."