The Union Party was a proslavery, unionist political party in the United States during the early 1850s. It was one of two main political parties in the slave states of Alabama, Georgia, and Mississippi, alongside the Southern Rights Party. The Georgia affiliate was known as the Constitutional Union Party. The party was organized to support the Compromise of 1850. While some figures such as Daniel Webster predicted a sweeping political realignment in which the Union Party would unite all those in favor of the Compromise measures, no national organization ever emerged. The party disbanded following acceptance of the Compromise by the Southern Rights leaders, with most former Unionists returning to their previous partisan allegiances.

Events following the Mexican–American War fueled rising tensions between the free and slave states, as proslavery fire-eaters threatened secession in response to the Wilmot Proviso. The crisis fractured the existing party system and produced an alliance between unionist Democrats and Whigs in the Lower South who sought to avert a civil war and defeat their intrapartisan rivals. Unionists were especially active in the 1851 elections, when Union parties elected 14 members to the U.S. House of Representatives and won governorships in Georgia and Mississippi. The acquiescence of the Southern Rights leaders to the Compromise after 1851 removed the need for a dedicated Union Party. Many Whigs who had supported the Union Party movement subsequently joined the Democratic Party, while most Union Democrats returned to their former political allegiance.

In states where Union parties were organized, Unionists supported preservation of the federal Union and opposed an independent Southern Confederacy. Ardently proslavery, they rejected secession as unconstitutional and ruinous to the interests of the slave states. Instead, they advocated a policy of conditional unionism wherein the slave states would remain loyal to the national government so long as the free states agreed to abide by the Compromise and abstain from any future attacks on slavery. While they opposed immediate secession, Unionists did not rule it out in the future should Southern demands go unheeded. Many who had been Unionists in the 1850s would go on to serve in the Confederate government during the Civil War, including Alexander H. Stephens, who served as vice president of the Confederacy from 1861 to 1865.

The Treaty of Guadalupe Hidalgo gained the provinces of Alta California and Nuevo México for the United States and opened the question of slavery's extension into the territory. Prior to ratification, the U.S. representative from Pennsylvania David Wilmot introduced an amendment providing as a "fundamental condition" of annexation that "neither slavery nor involuntary servitude shall exist in any part" of the Mexican Cession. The so-called Wilmot Proviso passed the House of Representatives but stalled in the Senate, where it became a serious point of contention between the free and slave states. The Proviso was a major issue in the 1848 United States presidential election, resulting in a split between the anti-extensionist Barnburner Democrats and the dominant faction of the Democratic Party, which advocated popular sovereignty. The Barnburners joined antislavery Conscience Whigs and abolitionists in the Free Soil Party, which polled 14 percent of the votes in the free states. The breach in the Democratic ranks allowed the Whig candidate, major general Zachary Taylor, to carry the critical state of New York and win the election while maintaining a noncommittal position on the territorial question.

Taylor planned to finesse the Proviso by granting immediate statehood to California and New Mexico, thus bypassing the territorial stage and avoiding the need for the national government to assume responsibility for slavery's exclusion from or expansion into the Southwest, but died before Congress could come to any determination. A compromise package introduced by the senator from Kentucky Henry Clay was defeated by the combined votes of Northern free-soilers and Southern fire-eaters. Following weeks of deadlock, the Compromise was shepherded through Congress item-by-item, with border state Whigs joining the Northern members to admit California as a free state, abolish the slave trade in the District of Columbia, and settle the boundary dispute between Texas and New Mexico, while Northern Democrats joined the Southern members to pass the Fugitive Slave Act of 1850 and organize the Utah and New Mexico territories without restrictions on slavery. Taylor's successor as president, Millard Fillmore, declared the Compromise measures a "final settlement" of sectional issues and committed U.S. soldiers to enforce the Fugitive Slave Act.

Clay's original omnibus proposal consisted of eight planks, subsequently enacted as individual items.

The Whig Party was a conservative political party in the United States during the mid-nineteenth century. It was the main opposition to the Democratic Party from 1834 until 1854. Following the party's unexpected defeat in the 1844 United States presidential election, some voices within the party called for a political realignment in which Whigs would join dissident Democrats in a new party. Taylor's nomination and election were seen as a step in this direction, in light of the general's perceived popularity with nativists, Democrats, and independent voters. The allocation of patronage in the new administration was indeed intended to build up a new "Taylor party" that would supplant the Whigs as the major opposition to the Democrats. Catastrophic Whigs losses in state and congressional elections held in the fall of 1849 put an end to the Taylor movement but not to discussion of a possible realignment among Whigs of various stripes.

Following his inauguration, Fillmore wrote to Hamilton Fish that he intended "to save the country [and] to save the Whig party, if possible." The president's strenuous support for the Compromise measures exacerbated divisions among Northern Whigs, however; intensified intraparty conflict led some pro-Compromise Whigs to conclude with Webster that "a new arrangement of Parties is unavoidable." During the winter of 1850–51, they increasingly looked to an alliance with pro-Compromise Democrats and "the creation of a great National Union Party" in answer to the problems presented by the sectional crisis.