1854–55 United States House of Representatives elections

States held the 1854–55 United States House of Representatives elections between August 4, 1854, and November 6, 1855 during President Franklin Pierce's term. Each state set a date to elect the 234 members and five non-voting delegates of the United States House of Representatives. In a critical defeat for the governing Democratic Party, opposition groups won more than 150 seats and control of the House, signaling the collapse of the Second Party System.

This midterm election was among the most disruptive in American history, auguring a political realignment. Both major parties, the Democratic Party and the Whig Party, rivals for roughly 20 years, lost critical voter support. Northern voters strongly opposed to the Kansas–Nebraska Act shifted sharply against the Democrats. The Whigs also lost seats despite the continuing Democratic Party split in New York, the most populous state, as the Whigs disintegrated nationally over the issue of slavery.

The elected majority temporarily coalesced as the Opposition Party. This transitional umbrella party included Whigs, Free Soil members, American Party members or Know Nothings, the People's Party of Indiana, Anti-Nebraska candidates, disaffected Northern Democrats, and members of the nascent Republican Party, which soon would absorb most of these factions and replace the Whigs to rival the Democrats.

Candidates opposed to the Democratic Party won widely in the Northern United States through November 1854. The American Party, ignoring slavery and opposing immigration (particularly by Catholics from Ireland and Germany) won seats from both major parties, but to the net loss of Democrats, in New England and the Southern United States from November 1854 into 1855.

Congress had passed the Kansas–Nebraska Act in May 1854 after aggressive sponsorship by the Pierce Administration and Democrats led by Senator Stephen Douglas, including radical pro-slavery legislators. With widely foreseen risks and immediately negative results, the Act discredited the Democratic Party, fueling new partisan and sectional rancor. The Act repealed the 1820 Missouri Compromise and triggered the violent Bleeding Kansas conflict, creating uncertainty on the Western frontier by abruptly making slavery potentially legal in territories originally comprising the northern portion of the Louisiana Purchase. Contemporary settlers of these territories then were expected to determine the status of slavery locally. This idea appealed to Democratic politicians and to some voters, but proved unworkable particularly in Kansas where more numerous Northern settlers and geographically closer Southern settlers violently disputed the status of slavery. Even some proslavery legislators and voters, particularly Southern Whigs, felt that repealing the Missouri Compromise was politically reckless, and that attempting to push slavery by law and force into territories where most settlers predictably were unlikely to want it could endanger slavery nationally, or even in the South. These fears proved prescient.

More than 21 representatives vied for the post of speaker of the House. After two months and 133 ballots, American Party representative Nathaniel Banks of Massachusetts, also a Free Soiler, defeated Democrat William Aiken of South Carolina by plurality, 103–100. To date, Banks is the only third party Speaker in House history.