1858–59 United States House of Representatives elections

The 1858–59 United States House of Representatives elections were held on various dates in various states between June 7, 1858, and December 1, 1859. Each state set its own date for its elections to the House of Representatives. 238 representatives were elected in the new state of Oregon, the pending new state of Kansas, and the other 32 states before the first session of the 36th United States Congress convened on December 5, 1859. They were held during President James Buchanan's term.

Winning a plurality for the first time, the Republicans benefited from multiple factors. These factors included the collapse of the nativist American Party, sectional strife in the Democratic Party, Northern voter dissatisfaction with the Supreme Court's March 1857 Dred Scott decision, political exposure of Democrats to chaotic violence in Kansas amid repeated attempts to impose slavery against the express will of a majority of its settlers, and a sharp decline in President Buchanan's popularity due to his perceived fecklessness. In Pennsylvania, his home state, Republicans made particularly large gains.

The pivotal Dred Scott decision was only the second time the Supreme Court had overturned an Act of Congress on Constitutional grounds, after Marbury v. Madison. The decision created apprehension in the Northern United States, where slavery had ceased to exist, that the Supreme Court would strike down any limitations on slavery anywhere in the United States with a ruling in Lemmon v. New York.

An inflammatory antislavery manifesto entitled The Impending Crisis of the South had attracted attention during the previous session of Congress. The book's author, Hinton Rowan Helper, attacked the planter class as a tyrannical oligarchy and called on poor white Southerners to overthrow them, "peacefully if we can, violently if we must." The Republicans financed a special edition of Helper's book for mass circulation which was endorsed by 68 Republican members of Congress, including the Republican House leader John Sherman. Following the elections, Republicans were the largest party in the House but lacked an overall majority and were forced to rely on Southern Oppositionists to organize the chamber. These Southerners refused to vote for Sherman to become Speaker of the United States House of Representatives due to his association with The Impending Crisis, resulting in the second-longest election in the history of the House of Representatives. Sherman eventually withdrew in favor of William Pennington, who was elected.

Republicans were united in opposing slavery in the territories and fugitive slave laws, while rejecting the abrogation of the Missouri Compromise, key aspects of the Compromise of 1850, the Kansas–Nebraska Act, and the Dred Scott decision. Though not yet abolitionist, Republicans openly derived a primary partisan purpose from hostility to slavery while furnishing a mainstream platform for abolitionism. None of the party's views or positions was new. However, their catalytic cohesion into a unified political vehicle, and the bold dismissal of the South, represented a newly disruptive political force.

Democrats remained divided and politically trapped. Fifteen Democratic members publicly defied their party label. Of seven Independent Democrats, six represented Southern districts. Eight Northern anti-Lecompton Democrats favored a ban on slavery in Kansas, effectively upholding the Missouri Compromise their party had destroyed several years earlier. Democrats lacked credible leadership and continued to drift in a direction favorable to the interests of slavery despite obviously widening and intensifying Northern opposition to the expansion of those interests. A damaging public perception also existed that President Buchanan had improperly influenced and endorsed the Dred Scott decision, incorrectly believing that it had solved his main political problem. Such influence would violate the separation of powers. The wide gap between Democratic rhetoric and results alienated voters, while defeat in the North and intra-party defection combined to make the party both more Southern and more radical.

Democrats lost seats in some slave states as the disturbing turn of national events and surge in sectional tensions alarmed a significant minority of Southern voters. Southern politicians opposing both Democrats and extremism, but unwilling to affiliate with Republicans, ran on the Southern Opposition Party ticket (not to be conflated with the Opposition Party of 1854).

For 11 states, this was the last full congressional election until the Reconstruction. Twenty-nine elected members quit near the end of the session following their states' secession from the Union, whose immediate motivation was the result of the presidential election of 1860.