Kansas-Nebraska and Bleeding Kansas

Stephen Douglas wanted a northern route for a transcontinental railroad, which meant organizing Nebraska Territory, which meant getting southern votes in Congress. His price for those votes was popular sovereignty: let the settlers of Kansas and Nebraska vote on slavery themselves. The catch was that both territories sat north of the 36 30 line, where the Missouri Compromise of 1820 had banned slavery outright. Douglas's bill repealed that ban. A rule that had held the Union together for thirty-four years was gone, replaced by a vote that both sides believed they could win.

Popular sovereignty sounds neutral, but it is not a referee, it is an invitation to a land rush. Pro-slavery Missourians crossed into Kansas to vote and settle; anti-slavery New Englanders funded by groups like the New England Emigrant Aid Company moved in to outnumber them. Each side built its own government: a pro-slavery legislature at Lecompton and a free-state government at Topeka, each claiming legitimacy. Ballot boxes got stuffed, elections got boycotted, and by 1856 the dispute had moved from votes to guns. Pro-slavery raiders sacked the free-state town of Lawrence; John Brown answered with the Pottawatomie Massacre, dragging five pro-slavery settlers from their homes at night and killing them. In Washington, Representative Preston Brooks caned Senator Charles Sumner nearly to death on the Senate floor after Sumner's speech attacking slavery, showing the violence had reached the Capitol itself.

The fallout broke the party system that had managed sectional conflict for decades. Northern Democrats who felt betrayed and northern Whigs who had no southern wing left to compromise with formed the Republican Party, built entirely around stopping slavery's expansion. That left a northern anti-slavery party facing a southern-dominated Democratic Party, the sectional alignment that would carry straight through to the election of 1860.