I. The Bitter Fruits of War
A. The Wilmot Proviso and the Expansion of Slavery
Most Americans agreed that the Constitution had left the issue of slavery to the individual
states to decide, but the issue of slavery in U.S. territories proved contentious.
In August 1846, Pennsylvania Democrat David Wilmot proposed that Congress bar
slavery in all lands acquired in the war with Mexico.
Regardless of party affiliation, Northerners, motivated by a variety of concerns, lined up
behind Wilmot's effort to stop the spread of slavery.
While the specter of new slave states alarmed most Northerners, the thought that slavery
might be excluded outraged almost all white Southerners.
Southern leaders understood the need for political parity with the North to protect the
South's interests, especially slavery, and in the nation's capital, the two sides squared off.
The House, dominated by northern states, passed the Wilmot Proviso; the Senate, with a
slave state majority, rejected it.
As a compromise, Senator Lewis Cass of Michigan proposed the doctrine of “popular
sovereignty”: letting the people who actually settled in the territories determine the fate
of slavery for themselves.
The plan's most attractive feature was its ambiguity about the precise moment when
settlers could determine slavery's fate; as long as the matter of timing remained vague,
popular sovereignty gave hope to both sides.
Congress failed to pass legislation related to slavery in the territories, and the unresolved
territorial question became an issue in the 1848 presidential election.
B. The Election of 1848
When President Polk chose not to seek reelection, the Democratic convention nominated
Lewis Cass, the man most closely associated with popular sovereignty; but the party
adopted a platform that avoided a firm position on slavery in the territories.
The Whigs, hoping to unite their divided party, nominated Mexican-American War hero
and slave owner Zachary Taylor; they, too, remained silent on the slavery issue.
In the summer of 1848, antislavery Democrats and antislavery Whigs founded the Free-
Soil Party, making slavery the central issue of the campaign.
The November election dashed the hopes of the Free-Soilers as Taylor won the election,
but the struggle over slavery in the territories had shaken the major parties badly.
C. Debate and Compromise
When Taylor assumed office, he championed a free-soil solution to the problem of
slavery in the western lands, encouraging California and New Mexico, both of which had
sizable antislavery majorities, to draw up state constitutions and apply for statehood as
quickly as possible.
Congress convened in December 1849, beginning one of the most contentious and most
significant sessions in its history.