The terms swing state, battleground state, and purple state are like three friends talking about the same thing, but from different perspectives. Each term describes a state that is closely divided between Democratic and Republican voters and could go either way in an election, but each points up different aspects of a political contest.
Battleground state is the oldest of the three terms, and we find examples of its use in its current political sense as early as 1860. The term underscores the grueling fight that can take place between opposing candidates who vie to win votes in a state with roughly equal support for both parties.
Purple state, the newest of the three terms, took on its current meaning in 2000, when political commentators settled on a consistent color scheme for depicting U.S. election maps: blue for a state in which Democrats have a majority; red where Republicans are in the majority; and purple (a blend of the colors blue and red) for a state with a nearly even split in voter allegiance. The term therefore emphasizes the fact that in a closely divided state, the mood and choices of voters may be hard to discern, even as they enter the polling booths on Election Day. A related but slightly different political use of the word purple is based on the observation that most states are politically and ideologically diverse: We live in a purple America and are not as polarized between red and blue as the news media would have us believe.
But the most frequently used term among the three is swing state, which seems to have gained traction during the politically tumultuous 1960s. The term underscores the drama of a close election, whose outcome can remain uncertain until the polls close. Famously close presidential contests (Kennedy–Nixon in 1960; Bush–Gore in 2000) have been determined by how a handful of swing states ultimately voted.
Although all three terms can be used interchangeably, they describe the drama of closely contested elections from distinctly different angles.