In presidential politics of the United States, a swing state (also, battleground state or purple state[note 1]) 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.
Origin of swing states
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 where the candidate who wins the most popular votes in a state wins all of that state's electoral votes. This way, presidential candidates have no incentive to spend time or resources in states they are likely to win or lose by a sizable margin.
Since a national campaign is interested in electoral votes, rather than the national popular vote, it tends to ignore states that it believes it will win easily; since it will win these without significant campaigning, any effort put into them is essentially wasted. A similar logic dictates that the campaign avoid putting any effort into states that it knows it will lose.
For instance, a Republican candidate (the more conservative of the two major parties) can expect to easily win many of the Southern states like Texas, Mississippi, Alabama, and South Carolina, which historically have a very conservative culture, very religious, and a more recent history of voting for Republican candidates. They could also expect to win states like Wyoming, Utah, Idaho and Nebraska, which share conservative values but have had a longer history of voting Republican. Similarly, the same candidate can expect to lose California, Vermont, Massachusetts, Oregon, Hawaii, Connecticut, Illinois, Rhode Island, and New York, traditionally liberal states, no matter how much campaigning is done in those states. The only states which the campaign would target to spend time, money, and energy in are those that could be won by either candidate. These are the swing states.
In Maine and Nebraska, the apportionment of electoral votes parallels that for Senators and Congressional Representatives. Two electoral votes go to the person who wins a plurality in the state, and a candidate gets one additional electoral vote for each Congressional District in which they receive a plurality. Both of these states have relatively few electoral votes (for the 2004 election, Maine had 4 and Nebraska had 5; the minimum is 3) and are usually not considered swing states (Maine is generally considered a Democratic-leaning state while Nebraska is typically thought to be a Republican state). Despite their different rules, only once has either state split its electoral votes: Nebraska in 2008, giving 4 votes to Republican John McCain and one to Democrat Barack Obama (who swept Maine).
In the 2004 elections, Colorado voted on Amendment 36, an initiative which would have allocated the state's electoral votes in proportion to the popular vote in the state. The initiative would have taken effect immediately, applying to the selection of electors in the same election. However, the initiative failed and Colorado remains under the winner-take-all system that is present in 48 states.
Determining swing states
The Oregon Daily Emerald cited University of Oregon political science professor Joel Bloom as mentioning three factors in identifying a swing state: "examining statewide opinion polls, political party registration numbers and the results of previous elections." The article also cites Leighton Woodhouse, co-director of "Driving Votes," as claiming that there is a general consensus among most groups regarding about 75 percent of the states typically thought of as swing states. 
In December 2008, Sean Quinn (of FiveThirtyEight.com) did a statistical analysis of the eight Mountain West states, and their change in vote from 2004 to 2008, thus concluding that they were the "new" swing region in the United States.
States where the election has a close result become less meaningful in landslide elections. Instead, states which vote similarly to the national vote proportions may be more useful. For example, the closest states in the 1984 presidential election were Minnesota and Massachusetts; however, a campaign strategy centered on them would be unlikely to be meaningful in the electoral college, as even if Democratic candidate Walter Mondale had won Massachusetts, he still would have been crushed. Rather, the "tipping-point state" which gave President Ronald Reagan his decisive vote was Michigan; Reagan won Michigan by 19 percentage points, quite similar to his national margin of 18.2%. Michigan would have been a more relevant state to the election results had the 1984 election come out closer. Similarly, Senator Barack Obama's narrow victory in Indiana in the 2008 election might overstate Indiana's importance as a swing state; Obama lost Indiana in the closer 2012 election, but was still reelected.
In 2012, the states of North Carolina, Florida, Ohio, and Virginia were decided by a margin smaller than 5 percentage points. However, none of these were the "tipping point" state; Mitt Romney could have won all of them and still lost the election. Rather, Colorado was the tipping point in 2012, as it was in 2008; Colorado voted for Obama by a margin of 5.4%, close to Obama's national margin of 3.9%. Pennsylvania, New Hampshire, and Iowa all had comparable margins to Colorado, and were heavily campaigned in during the 2012 election.
Historical swing states
The swing states of Ohio, Connecticut, Indiana, New Jersey and New York were key to the outcome of the 1888 election, Likewise, Illinois and Texas were key to the outcome of the 1960 election. Florida and New Hampshire were key to the 2000 election. Ohio was the key to the 2004 election. Ohio has gained a reputation as a swing state since the 1980s, and last voted against the person declared president in the 1960 election.
Criticism and proposed reform
Those in favor of a national popular vote as the method for electing the president argue that the electoral system gives swing states arbitrarily large power in determining the result of an election, and therefore receive an undeservedly large proportion of attention and campaign funds. Although a constitutional amendment would be the simplest form for changing the rules, the difficulty of the task has led to a proposal to make States enter a National Popular Vote Interstate Compact. The reform is hotly debated within State assemblies, because it would arguably change the nature of the election.
- A purple state is referencing the combination of red and blue, in reference to Red states and blue states.
- "Portrait of a swing State", Meghann Cuniff, Oregon Daily Emerald, October 4, 2004.
- Quinn, Sean (2008-12-03). "Politics Done Right: The Mountain West: America's New Swing Region". FiveThirtyEight. Retrieved 2012-11-08.
- Silver, Nate (2012-04-27). "Arizona Is (Probably) Not a Swing State". The New York Times. Retrieved 2013-06-06.
- Silver, Nate (2012-11-08). "As Nation and Parties Change, Republicans Are at an Electoral College Disadvantage". Retrieved 2013-06-06.
- "1888 Overview" p.4, HarpWeek.
- "Daley Remembered as Last of the Big-City Bosses", David Rosenbaum, New York Times, April 21, 2005.
- Trolling the Campuses for Swing-State Votes, Julie Salamon, "The New York Times", October 2, 2004
- Game Theory for Swingers, Jordan Ellenberg, "Slate.com", October 25, 2004
- The Critical 2012 Swing States
- Battleground States 2008 via the Washington Post
- Swing State Ohio Documentary
- Swing State feature documentary project
- Guide to the 2004 swing states from Slate
- Battleground states from Democracy in Action site hosted by George Washington University
- How close were Presidential Elections? Influential States - Michael Sheppard
- The Bush campaign memo detailing its look at the swing states (PDF file)