The final days of a close race for the White House, like the one now unfolding between President Obama and Mitt Romney, are always loud, tense and messy, but most end with a clear result.
What if this year is different, and Nov. 6 is only the beginning of the chaos?
What if it ends in a tie?
An Electoral College tie - with each man getting 269 votes instead of the 270 needed to win - is possible, analysts say, a result that would unleash enough political and legal disorder to make people yearn for the tamer controversy over "hanging chads" in the 2000 Florida recount.
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 that phenomenon: faithless electors.
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Given the expected composition of 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.
Brown said the resulting conflict 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 meet in the state capitals in December to ratify the popular vote in their states, but here's the twist: They may not be 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 Democrats regain the House Nov. 6.
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 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 runner-up John Quincy Adams.
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 vote.
