Note to Mitt Romney and Barack Obama: Don't forget about 2000.
That was a year that a third-party candidate made a difference in a presidential election, when Ralph Nader drew nearly 3 million votes and helped tip the race toward then-Texas Gov. George W. Bush and away from Democratic Vice President Al Gore.
But in any close election -- as this year's presidential contest between Democratic Presidential Barack Obama and Republican presidential nominee Mitt Romney is expected to be -- political analysts warn it could happen again.
"Third parties can play 'spoiler' roles ... but they do play a major role," said Allan Saxe, an associate political science professor at the University of Texas at Arlington. "The closer the election, the more influence third and other parties have."
Nader isn't on the presidential ballot this year, but Libertarian Gary Johnson and Green Party nominee Jill Stein are -- not to mention Virgil Goode, a former Virginia congressman who is expected to end up on several states' ballots as a Constitution Party presidential candidate, just not in Texas.
And that, political observers say, means anything can happen in November.
"Third parties have not influenced the outcome of presidential elections since 2000," said Lyle Brown, a professor emeritus of political science at Baylor University in Waco.
That year, many say Nader -- then a Green Party candidate -- played the spoiler, likely helping Bush win the White House.
Florida -- where Nader picked up nearly 100,000 votes and Gore lost to Bush by 537 votes -- was a deciding factor. In that razor-thin presidential race, Florida's electoral votes gave Bush the presidency.
"The votes that Ralph Nader received in several states in 2000 would have been enough to give Al Gore an electoral college victory," said Adam Schiffer, a political science professor at Texas Christian University in Fort Worth. "Had even one-fifth of Nader's voters voted for Gore instead, Florida would not have needed a recount."
Nader earlier this year said that third-party candidates play an important role in the electoral process.
"I think it's competition," he told MSNBC earlier this year. "I think it's new agendas, new ideas, that are supported by a large number by the American people.
"And I think, above all, it respects the voters by raising their expectation level," he said. "That's the history of small parties."
Texas billionaire Ross Perot, running as an independent, affected the 1992 presidential race -- which pitted then incumbent Republican President George H.W. Bush and Democratic then-Arkansas Gov. Bill Clinton.
Clinton won with nearly 45 million votes to Bush's more than 39 million votes. Perot picked up more than 19 million votes in that race.
"Many observers credit Ross Perot with tipping the 1992 election to Clinton, but the best scholarly analyses of polling data dispute that conclusion," Schiffer said.
Johnson, a former Republican governor of New Mexico, is expected to influence the presidential race, particularly in his home state.
Goode, who served six terms in Congress, is on the ballot in more than a dozen states. Political observers say Goode could affect the presidential vote at least in his home state of Virginia, where he recently secured a ballot spot.
And Stein, a Harvard-educated doctor from Massachusetts who recently was arrested during a Philadelphia sit-in, may appeal to a different faction of voters.
If third-party candidates gain enough votes, that might be enough to tip certain states -- particularly Virginia, where Obama has said a win could propel him to overall victory, and which Romney has said is key in his own campaign strategy -- to a candidate who might not have otherwise won.
"Of course, the closer the election, the more influence third and other parties have," Saxe said. "In various states this year, there likely will be many parties and independent candidates, and usually [it's] no big deal. But it could add up to influence a state's electoral vote."