While he was in Congress he voted repeatedly for the principle of the Wilmot Proviso.
He began to take an active part in politics in 1844, and in 1845-1847 was a Democratic representative in Congress, where he advocated the Wilmot Proviso.
In the third place, the rejection of the Wilmot Proviso and the acceptance (as regards New Mexico and Utah) of "Squatter Sovereignty" meant the adoption of a new principle in dealing with slavery in the territories, which, although it did not apply to the same territory, was antagonistic to the Missouri Compromise of 1820.
Bradford, who, as Miss Wilmot, had resided with the princess between 1803 and 1808, and had suggested their preparation.
Later when the conservatives accepted the annexation of Texas and the radicals supported the Wilmot Proviso the split became irrevocable.