The Republican Party, also known as the GOP (Grand Old Party), is one of the two major contemporary political parties in the United States. It emerged as the main political rival of the then-dominant Democratic Party in the mid-1850s, and the two parties have dominated American politics ever since. It was founded in 1854 by anti-slavery activists who opposed the Kansas–Nebraska Act, an act which allowed for the potential expansion of chattel slavery into the western territories of Kansas and Nebraska.

In 1854 it was founded in the Northern United States by forces opposed to the expansion of slavery, ex-Whigs, and former Soil Party members. The Republican Party quickly became the principal opposition to the dominant Democratic Party and the briefly popular Know Nothing Party. At the Republican Party's first National Convention in 1856, held at Musical Fund Hall in Philadelphia, the party adopted a national platform emphasizing opposition to the expansion of slavery into the free territories.