In United States presidential politics, a swing state (also, battleground state or purple state) is a state in which no single candidate or party has overwhelming support in securing that state's electoral college votes. Such states are targets of both major political parties in presidential elections, since winning these states is the best opportunity for a party to gain electoral votes. Non-swing states are sometimes called safe states, because one candidate has strong enough support that he or she can safely assume that he or she will win the state's votes.

In U.S. presidential elections, the Electoral College system allows each state to decide the method by which it awards electors. Since in most states the legislature wants to increase the voting power of the majority, all states except Maine and Nebraska (explained below) use a winner-take-all system (as opposed to a majority electoral system) where the candidate who wins the most popular votes in a state wins all of that state's electoral votes. Under this system no advantage is gained by winning more than a plurality of the vote, nor is there any advantage gained by winning additional votes in a state that will still be lost. In other words, Presidential candidates have no incentive to spend time or resources in states they are likely to win or lose by a sizable margin.