In a tie scenario, the Constitution says the House of Representatives would choose the next president, and the Senate would choose the vice president - provided no members of the Electoral College desert their candidate. There's a term for this phenomenon: faithless electors.
Given the expected composition of the next Congress, the result could be an administration divided against itself: President Romney and Vice President Biden.
"A tie is logically plausible, but unlikely," said Lara M. Brown, a Villanova University political scientist who has written on the Electoral College. "It's tough to pull off an inside straight. The math would have to line up just right."
It could happen, for example, if the battle for swing states ends with Romney winning Iowa, Florida, New Hampshire, North Carolina, and Ohio, and Obama taking Colorado, Virginia, and Wisconsin.
If it did happen, the popular uproar could be potentially destabilizing, Brown said, with the supporters of the losing candidate questioning the legitimacy of the winner.
She said that might, in turn, trigger efforts to abolish the Electoral College by constitutional amendment - upsetting "the framers' purpose in creating a balance, where the more populous states could not just outweigh all the other states."
Another, more familiar scenario: One candidate could win the popular vote, but lose in the Electoral College, as when Republican George W. Bush beat Democrat Al Gore 12 years ago after a bitter legal fight that ended when the U.S. Supreme Court awarded Florida to Bush.
Presidential elections are not decided by a winner-take-all national vote, but in state-by-state contests to get a majority of 538 electoral votes. The votes are apportioned mostly by population, except that each state gets a minimum of three.
At one time, the electoral votes totaled an odd number (535), rendering a tie unlikely. But in 1961 the Constitution was amended to give the District of Columbia three electoral votes - bringing the total to 538.
Slates of electors pledged to each candidate - typically, officeholders and party insiders - meet in the state capitals in December to ratify the popular vote in their states, but here's the twist: They are not, in most cases, bound to follow the people's will. (Some states have laws requiring it; most do not.)
Then, when the new Congress convenes in January, electoral votes are tallied in Washington. In the case of a tie, the 12th Amendment says that the House would decide the winner, with each state's congressional delegation allotted a single vote.
Republicans now hold a majority in 33 state delegations, to 16 delegations for the Democrats (Minnesota's is split evenly between Ds and Rs), meaning a likely Romney victory - that is, unless there is a reversal of stunning proportions at the polls Nov. 6 and Democrats regain control of the House.
A presidential election has ended with a tie before - but not in two centuries. The House decided the 1800 election, choosing Thomas Jefferson over Aaron Burr in February 1801, after each had won 73 electoral votes.
In 1824, with four candidates in the race, nobody won a majority in the Electoral College - Andrew Jackson had the most popular and electoral votes, but the House later anointed runner-up John Quincy Adams. (Jackson supporters believed Adams and one of the other candidates, Henry Clay, struck what historians call "the corrupt bargain" to swing the election. Adams named Clay his secretary of state.)
If an Electoral College tie occurs this time, chaos could await the new Congress in 2013 - if it got that far. Since electors are not bound to follow the popular vote except in a few states, all it would take to get a winner would be for one "faithless elector" to change his or her vote.
Experts say the electors could be subject to an unprecedented lobbying campaign.
"The possibilities for chicanery are scary," said Robert Alexander, a political scientist at Ohio Northern University who has written a book about the Electoral College based on surveys of the 2000, 2004, and 2008 electors.
"If it's really close or tied, they would be lobbied intensively," Alexander predicted. "Most electors are party loyalists and do follow the herd, but sometimes they waver and wrestle with their consciences."
Two Bush electors from 2000 said they considered voting for Gore because he had won the most popular votes, Alexander said. Democratic consultants, led by Bob Beckel, had mounted a lobbying campaign to pressure electors after the Supreme Court decision.
About 10 percent of electors in 2004, when Bush was reelected over Democrat John Kerry, told Alexander's researchers they considered voting for someone other than the candidate to whom they were pledged - and one anonymous Minnesota elector did cast his ballot for John Edwards, Kerry's running mate. In 2008, as many as 11 percent of electors thought about switching.
A group of "birthers," convinced that Obama was not born in the United States, contacted most of his electors, asking them to block his election, Alexander found.
None deserted their pledge.
"They were told, 'You're the last line of defense for the Constitution,' " Alexander said. "It was crazy - I thought, 'This is a movie.' It didn't seem real."
Contact Thomas Fitzgerald
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