The Republican Party, also known as the Grand Old Party (GOP), is a conservative and right-wing political party in the United States. It emerged as the main rival of the Democratic Party in the 1850s, and the two parties have dominated American politics since then.

The Republican Party was founded in 1854 by anti-slavery activists opposing the Kansas–Nebraska Act and the expansion of slavery into U.S. territories. It rapidly gained support in the North, drawing in former Whigs and Free Soilers. Abraham Lincoln's election in 1860 led to the secession of Southern states and the outbreak of the American Civil War. Under Lincoln and a Republican-controlled Congress, the party led efforts to preserve the Union, defeat the Confederacy, and abolish slavery. During the Reconstruction era, Republicans sought to extend civil rights protections to freedmen, but by the late 1870s the party shifted its focus toward business interests and industrial expansion. In the late 19th and early 20th centuries, it dominated national politics, promoting protective tariffs, infrastructure development, and laissez-faire economic policies, while navigating internal divisions between progressive and conservative factions. The party's support declined during the Great Depression, as the New Deal coalition reshaped American politics. Republicans returned to national power with the 1952 election of Dwight D. Eisenhower, whose moderate conservatism reflected a pragmatic acceptance of many New Deal-era programs.

Following the civil rights era, the Republican Party's use of the Southern strategy appealed to many white voters disaffected by Democratic support for civil rights. The 1980 election of Ronald Reagan marked a major realignment, consolidating a coalition of free market advocates, social conservatives, and foreign policy hawks. Since 2009, internal divisions have grown, leading to a shift toward right-wing populism, which ultimately became its dominant faction. This culminated in the 2016 election of Donald Trump, whose leadership style and political agenda—often referred to as Trumpism—reshaped the party's identity. By the 2020s, the party has increasingly shifted towards illiberalism. In the 21st century, the Republican Party's strongest demographics are rural voters, White Southerners, evangelical Christians, men, senior citizens, and voters without college degrees.

On economic issues, the party has maintained a pro-capital attitude since its inception. It currently supports Trump's mercantilist policies, including tariffs on imports on all countries at the highest rates in the world while opposing globalization and free trade. It also supports low income taxes and deregulation while opposing labor unions, a public health insurance option and single-payer healthcare. On social issues, it advocates for restricting abortion, supports tough on crime policies, such as capital punishment and the prohibition of recreational drug use, promotes gun ownership and easing gun restrictions, and opposes transgender rights. Views on immigration within the party vary, though it generally supports limited legal immigration but strongly opposes illegal immigration and favors the deportation of those without permanent legal status, such as undocumented immigrants and those with temporary protected status. In foreign policy, the party supports U.S. aid to Israel but is divided on aid to Ukraine and improving relations with Russia, with Trump's ascent empowering an isolationist "America First" foreign policy agenda.

In 1854, the Republican Party emerged to combat the expansion of slavery into western territories after the passing of the Kansas–Nebraska Act. The early Republican Party consisted of northern Protestants, factory workers, professionals, businessmen, prosperous farmers, and after the Civil War also of black former slaves. The party had very little support from white Southerners at the time, who predominantly backed the Democratic Party in the Solid South, and from Irish and German Catholics, who made up a major Democratic voting block. While both parties adopted pro-business policies in the 19th century, the early GOP was distinguished by its support for the national banking system, the gold standard, railroads, and high tariffs. The party opposed the expansion of slavery before 1861 and led the fight to destroy the Confederate States of America (1861–1865). While the Republican Party had almost no presence in the Southern United States at its inception, it was very successful in the Northern United States, where by 1858 it had enlisted former Whigs and former Free Soil Democrats to form majorities in nearly every Northern state.

With the election of its first president, Abraham Lincoln, in 1860, the party's success in guiding the Union to victory in the Civil War, and the party's role in the abolition of slavery, the Republican Party largely dominated the national political scene until 1932. In 1912, former Republican president Theodore Roosevelt formed the Progressive Party after being rejected by the GOP and ran unsuccessfully as a third-party presidential candidate calling for social reforms. The GOP lost its congressional majorities during the Great Depression (1929–1940); under President Franklin D. Roosevelt, the Democrats formed a winning New Deal coalition that was dominant from 1932 through 1964.

After the Civil Rights Act of 1964 and the Voting Rights Act of 1965 during the civil rights movement, the party pursued the electoral Southern strategy to increase political support among white voters in the South by appealing to racism against African Americans. As a result, the party's core base shifted in a political realignment with the Southern states becoming more reliably Republican in presidential politics as part of a new "Solid South". White voters increasingly identified with the Republican Party after the 1960s. The following decades saw the party reduce its overt racial appeals to win moderate voters. Following the Supreme Court's 1973 decision in Roe v. Wade, the Republican Party opposed abortion in its party platform and grew its support among evangelicals. The Republican Party won five of the six presidential elections from 1968 to 1988. Two-term President Ronald Reagan, who held office from 1981 to 1989, was a transformative party leader. His conservative policies called for reduced social government spending and regulation, increased military spending, lower taxes, and a strong anti-Soviet foreign policy. Reagan's influence upon the party persisted into the 21st century.

Since the 1990s, the party's support has chiefly come from the South, the Great Plains, the Mountain States, and rural areas in the North. There have been 19 Republican presidents, the most from any one political party.