In United States politics, a swing state (also known as battleground state, toss-up state, or purple state) is any state that could reasonably be won by either the Democratic or Republican candidate in a statewide election, most often referring to presidential elections, by a swing in votes. These states are usually targeted by both major-party campaigns, especially in competitive elections. Meanwhile, the states that regularly lean to a single party are known as "safe states" (or more specifically as "red states" and "blue states" depending on the partisan leaning), as it is generally assumed that one candidate has a base of support from which a sufficient share of the electorate can be drawn without significant investment or effort by the campaign. In the 2024 United States presidential election, seven states were widely considered to be the crucial swing states: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania, and Wisconsin.

Due to the winner-take-all method that most states use to determine their presidential electors, candidates often campaign only in competitive states, which is why a select group of states frequently receives a majority of the advertisements and candidate visits. The battlegrounds may change in certain election cycles and may be reflected in overall polling, demographics, and the ideological appeal of the nominees.

In United States presidential elections, each state is free to decide the method by which its electors to the Electoral College will be chosen. To increase its voting power in the Electoral College system, every state, with the exceptions of Maine and Nebraska, has adopted a winner-take-all system, where the candidate who wins the most popular votes in a state wins all of that state's electoral votes.

The expectation was that the candidates would look after the interests of the states with the most electoral votes. However, in practice, most voters tend not to change party allegiance from one election to the next, leading presidential candidates to concentrate their limited time and resources campaigning in those states that they believe they can swing towards them or stop states from swinging away from them, and not to spend time or resources in states they expect to win or lose.

Because of the electoral system, the campaigns are less concerned with increasing a candidate's national popular vote, tending instead to concentrate on the popular vote only in those states which will provide the electoral votes it needs to win the election, as many successful candidates have lost the popular vote but won the electoral college.

In past electoral results, Republican candidates would have expected to easily win most of the mountain states and Great Plains, such as Idaho, Wyoming, the Dakotas, Montana, Utah, Kansas, Oklahoma, and Nebraska, most of the South, including Alabama, Mississippi, Louisiana, Arkansas, Tennessee, Kentucky, South Carolina, Missouri, Texas, and West Virginia, as well as Alaska. Democrats usually take the Mid-Atlantic states, including New York, New Jersey, Maryland, Virginia, and Delaware, New England, particularly Vermont, Massachusetts, Rhode Island, and Connecticut, the West Coast states of California, Oregon, Washington, Hawaii, and the Southwestern states of Colorado and New Mexico, as well as the Great Lakes states of Illinois and Minnesota.

However, some states that consistently vote for one party at the presidential level occasionally elect a governor of the opposite party; this is currently the case in New Hampshire, Vermont and Virginia which have Republican governors, as well as in Kentucky and Kansas, which currently have Democratic governors. Even in presidential election years, voters may split presidential and gubernatorial tickets. In 2024, this occurred in three states: North Carolina, Vermont and New Hampshire. Vermont and New Hampshire both elected Republican governors even as Democrat Kamala Harris won both states. North Carolina, despite voting for Republican Donald Trump, elected a Democratic governor. North Carolina has elected a Democratic governor in each concurrent gubernatorial election where Donald Trump was the Republican presidential nominee, by a notably wide margin in 2024.

In Maine and Nebraska, the apportionment of electoral votes parallels that for U.S. senators and representatives. Two electoral votes go to the candidate who wins the plurality of the vote statewide, and a candidate gets an additional electoral vote for each congressional district in which they receive a plurality. Both of these states have relatively few electoral votes – a total of 4 and 5, respectively. Nebraska has split its votes since 1992, and Maine has done so since 1972. Each state has split its electoral votes only thrice since implementation: all three times Maine's second district gave one vote to Donald Trump, in 2016 (won), 2020 (lost) and 2024 (won); while Obama in 2008 (won), Biden in 2020 (won), and Harris in 2024 (lost) obtained the Nebraska's second district vote in their respective races.