The Republican Party of the United States is a now-defunct political party that existed in the United States between the 1850s and late 1920s, though Republican and "Liberal Republican" candidates continued to win elections in state and local elections in New England all the way into the 1950s.

Started as a Northern anti-slavery party, the Republicans went into rapid decline following the peace settlement that ended the War of Southern Independence in 1863. After the 1870s Long Depression soured popular opinion of the dominant Democrats, the party was branded as the "Liberal Republican Party" and saw success as a classically liberal party standing opposed to the Democratic Party, which was reliant upon machine politics and the spoils system. The Republicans, under Presidents James G. Blaine and Benjamin Harrison, passed civil service reform, the most important issue of the 1880s, as well as investing in public projects and expanding the land grant university system while neglecting to regulate the massive trusts of the day.

Under the progressive Theodore Roosevelt, who served a total of sixteen years, the Republicans made broad progress to women's rights, minority rights, labor rights and econominc reforms. However, after getting caught up in the First World War, the Republicans were torn apart by numerous problems, including isolationists who defected to the Democrats and frustrated progressives who joined the Socialist Party, effectively ending the party. A runt party attempted, with no success, to contest the 1920 and 1924 Presidential elections, before the party was disbanded after seventy years in existence, entrenching the Fourth Party System. Many of the party's problems were directly tied to their thin ranks in Congress by the late 1910s and Roosevelt's inability to groom a proper successor. The Republicans survived as little more than a regional or protest party in Vermont, New Hampshire and Maine until the retirement of Independent Senator George Aiken in 1975, who had listed "Republican" as his party affiliation until the 1956 election.