A swing state (also, battleground state) in United States presidential politics is a state in which no candidate has overwhelming support, meaning that any of the major candidates have a reasonable chance of winning the 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 votes. Non-swing states are sometimes called safe states, because one candidate has strong enough support that they can safely assume they will win the state's votes.

Origin of swing states
The actual procedures for deciding which states are swing states in any particular election varies across campaigns and across disciplines. Many political scientists use historical voting patterns: the more often a state has been won by candidates of one party in the past, the more likely it is to vote for that party in the future.

Determining swing states
The swing states of Connecticut, Indiana, New Jersey and New York were key to the outcome of the 1888 election.

Historical swing states
The following states are grouped by geographic regions: (in brackets are electoral votes at stake in the 2008 election, and their 2004 vote)

Swing States as of 2007

New Hampshire (4-D): Once very reliably Republican, New Hampshire became a swing state in the 1990s. Republicans still have somewhat of an edge in statewide elections, however the Democrats took control of the state legislature and both Congressional seats in 2006. The New Hampshire Republican Party tends to be more socially liberal than the national party, and as a result their behavior in national elections is harder to determine.
Pennsylvania (21-D): Pennsylvania is famously described by Democratic strategist James Carville as "Pittsburgh to the west, Philadelphia to the east, and Alabama in the middle." Northeast

Arkansas (6-R): Although a conservative state in the heart of the Bible Belt, the Democratic Party is a powerful force in Arkansas and Democrats tend to have a comfortable advantage in statewide races. Presently, the Governor, both U.S. Senators, and 3 out of 4 of the Arkansas' House members are Democrats, and Democrats control the state legislature by a large margin. The Arkansas Democratic Party tends to be more conservative than the national party, however, and as a result voters there tend to be open to Republican Presidential candidates. Though favorite son Bill Clinton won Arkansas easily both times he ran, Arkansas gave their electoral votes to George W. Bush in 2000 and 2004 by a fairly large margin.
Virginia (13-R): No Democratic presidential candidate has won Virginia since Lyndon Johnson's landslide victory in 1964, and it was the only Southern state that went Republican in 1976. Virginia is no longer as reliably Republican as it once was, as evidenced by Democrat Tim Kaine's victory in 2005 for the Governor's Mansion and Jim Webb's narrow victory in the 2006 Senate race against incumbent Republican George Allen.
Florida (27-R): The outcome of the 2000 Presidential Election hung on a margin of roughly 500 votes in this state, and the fierce legal battles that ensued. Florida's electorate is balanced by heavily Democratic large cities like Miami and sparser, more Republican areas (the Florida Panhandle in this case). Republicans have been winning handily in statewide elections lately, however, the large Hispanic vote near Tampa and Orlando provide Democrats an edge, but the Cuban-American vote is crucial near Miami; their votes gave an edge to George W. Bush over Al Gore in 2000. South

Minnesota (10-D): Minnesota's transformation into a swing state is an ironic one, given how fervently Democratic the North Star State once was - it was the only state in the country that did not vote for Ronald Reagan in 1984. A strong tradition of populism and labor unions made it difficult for Republicans to have any real success there until recently.
Wisconsin (10-D): Wisconsin has narrowly gone to Democratic candidates for the past several years, which is somewhat ironic considering that the Republican Party was founded there. The Republicans lost their advantage in Wisconsin in the late 19th century when perceived nativist sentiments - particularly the Bennett Law - alienated the state's large German-American population. Wisconsin has a strong progressive tradition, and elected the country's only current openly lesbian U.S. Congresswoman.
Iowa (7-R): Al Gore won Iowa in 2000 by a razor-thin margin, and George W. Bush did the same four years later. The state's highly influential primary makes Iowa the political holy grail of Republicans and Democrats alike.
Missouri (11-R): Missouri is geographically situated where the South, the Midwest, and the Great Plains meet, and is in many ways a microcosm of the entire country. Missouri has voted for the winner of nearly every Presidential election since 1904 (they got it wrong only once, in 1956), and voters there have proven themselves to be an effective gauge of the national mood.
Ohio (20-R): "I think 2008 is very likely to be a hotly contested race in Ohio," stated Eric Rademacher, director of the University of Cincinnati's Ohio Poll, for the Cincinnati Enquirer.
Michigan (17-D): Michigan has generally tended to lean Democratic. One of the country's biggest centers of manufacturing, labor unions inevitably come into play, and the economic hard times the state has fallen on recently will no doubt be a major issue for the Great Lakes State in 2008. Republican strength tends to be primarily in the western peninsula of the state. Midwest

Colorado (9-R): A reliable GOP stronghold, Colorado has started to steer towards the center, where moderate stances have come to prevail. With the victories of Ken Salazar to the U.S. Senate in 2004, Bill Ritter to the Governor's Mansion in 2006 along with an additional U.S. House seat pick-up that same year, Demcorats are finding themselves in a better position than before. Large Hispanic populations with strong penchanct for populist themes makes this a true battleground states; however Republicans have a 100,000 registration edge against the Democrats.
New Mexico (5-R): A classic swing state, personalities trump party affiliation in this Western state. New Mexico is truly politically divided, with registration amongst Democrats and Republicans nearly equal and the existence of a strong Independent voting bloc. The state went to Al Gore in 2000 by a mere 400 votes while George W. Bush carried it by a margin of 5000 votes in 2004.
Nevada (5-R): Usually considered a "fly-over" state due to its proximity to California with candidates looking for more exposure on the coast, the Silver State is once again looking like a strong swing state. Mormon populations make a decisive vote for the GOP while the presence of strong labor unions and Hispanic voters sway them towards the Democrats in areas like Las Vegas and Reno. A GOP bastion, the state has turned its tide for more populist themes.
Oregon (7-D): A Democratic-leaning state, the belief of anti-big government along with strong opposition to central government control, makes Oregon a GOP favorite; however, intense beliefs in civil liberties and liberal ideology on social issues force it to side with the Democrats. The state has gone to the Democrats from the 1988 election onward. West/Pacific
Overall, the candidates spend a significant amount of time in the Midwest, as most of the electoral votes, a grand total of 75 electoral votes are at stake. Most of them have been inclined to vote for the Democrats, but by a small margin, making them volatile to switch parties.
As for the "Big 3": Pennsylvania, Florida and Ohio; all the candidates spend a huge chunk of their time there in order to offset any surprises that might erupt on election day. These states also provide a grand total of 68 electoral votes alone, therefore, making them a huge attraction.
Historically, no Republican candidate has won the White House without winning in Ohio; the winner of Ohio has been the winner of the general election since 1960.