Crisis.

Neither the Wilmot Proviso nor the Treaty of Guadelupe Hidalgo settled what was becoming the most convulsive political question of the day: whether slavery would spread into territory gained from Mexico. As the presidential election of 1848 approached, many Americans looked to the electoral system to decide the matter, and since President Polk decided not to seek reelection, the field was wide open.

The Liberty Party.

First to make a formal nomination was the abolitionist Liberty Party. Although riven with strife over how radical a program to pursue, the political abolitionists of the Liberty Party agreed with the principle of the Wilmot Proviso. The party convention endorsed the Proviso and named John P. Hale, a former Democratic senator from New Hampshire, for president.

Calhoun and "Southern Rights."

On the other side of the spectrum, John C. Calhoun staked out a...