Yesterday, in the late 1840s, I talked about the outbreak of the Mexican War. Like Texas annexation the votes on it showed a pronounced sectional divide. It did not reach the extremes of the 1850s, but did mark another step away from an era where both parties, which had constituencies in North and South alike, worked to avoid being either the slavery party or the antislavery party.
Always unstable, that settlement could not last forever. Texas strained it, but the popularity of annexation in general somewhat ameliorated the strain. Much the same happened with the votes on the Mexican War. Like in the Second World War, planning for the peace began during the fighting. Would the new territories of the Mexican Cession be slave or free?
Representative David Wilmot (D-PA) answered on August 8, 1846, in a rider on an appropriations bill:
Provided, That, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys herein appropriated, neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted.
For a Democrat, a member of the party of expansion and the party more successful at marginalizing its antislavery wing, this was radical. It might not have happened, had Polk not alienated Northern Democrats with a low tariff (harming Northern manufacturing interests) and by compromising with the United Kingdom over the division of the Oregon Country at the same time as he favored extreme Southern expansion. Going further back, Southern Democrats denied Van Buren nomination for a second term in 1844 by bringing back a disused rule requiring a ⅔ majority of the convention.
Wilmot’s proviso joined the bill with a 83-64 sectional majority. Attempts to forestall it with an extension of the Missouri Compromise (89-54) and to kill the entire bill (94-78) failed and the bill went to the Senate, where the sections stood evenly split. Senate Democrats planned to strip the Proviso and throw the bill back to the House. Senator John Davis (Whig-MA) tried to hold the floor until the last moment, leaving no time for the House to re-vote and thus forcing the Senate to take the bill as-is. But the official House and Senate clocks differed by eight minutes. When he called a vote, the House had already adjourned and the bill died.
The Proviso returned in 1847, this time expanded to include all territory on the North American continent. Representative Stephen Douglas (D-Il), Lincoln’s famous debate partner, countered with another proposal to extend the Missouri Compromise, failing 109-82. The strengthened Proviso passed the House again only to be defeated by the Senate. When the Proviso-free bill returned to the House, every Northern Whig supported the Proviso still, but 22 Northern Democrats voted with the South. The Proviso came up one last time, as an amendment to the Treaty of Guadalupe Hidalgo ending the war. Now a senator, Stephen Douglas joined with other Northern Democrats and the South in defeating it.
But all of this left question of slavery in the Mexican Cession unanswered.
What began with Texas annexation as a partisan, but also sectional divide on expansion strategy had become a sectional, but also partisan divide on slavery expansion. The division hit the Whig party hardest, but the Northern, frustrated Van Buren Democrats, after failing to get a pro-Proviso plank into the party platform in 1848, came to see their party as dominated by Slave Power and began looking for a new home.