For the United States government, the addition of this new territory was political kryptonite. Both Northerners and Southerners were convinced that the opposite region was conspiring to limit their economic opportunities in the West. During the war, Congressman David Wilmot of Pennsylvania introduced a resolution in the House that would prohibit slavery in any territory gained from the conflict. The reaction to the Wilmot Priviso showed just how big the sectional divide in the country was becoming, since party lines broke down entirely. Northerners, Whig, and Democrat alike voted for the Wilmot Proviso, and Southerners, Whig, and Democrat alike voted against it. Ultimately, the proviso passed in the House was defeated in the Senate. And then gold was discovered in California, just before the end of the war, sending hordes of prospectors West and making statehood for California an urgent issue that would soon upset the balance of power between free and slave states in Congress. In other words, we can draw a direct line from the Mexican War to the breakdown of the second party system, which was replaced by a solidly ____________and a ______________, and from there to the Civil War.

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