If all goes smoothly in Tuesday's election, Americans will know who their president is by the end of the night. But if there are complications, there could be weeks of legal wrangling that could have a number of outcomes, the most intriguing of which is an inauguration where Mitt Romney, the Republican, would be sworn in alongside Joe Biden, the current Democratic vice-president.
The prospect may seem unlikely. But the race between Romney and Barack Obama is so close that it could happen if there is a draw in the electoral-college system used to appoint the president. Under the US constitution's 12th amendment, a 269-269 draw would require the election to be decided by the newly elected House of Representatives.
The Republicans are expected to retain their majority in the House after Tuesday and would naturally choose Romney as president. But the Senate would choose the vice-president, and that body is expected to stay under Democratic control. The 12th amendment has only been invoked once, in 1824, when the electoral college was split between four candidates.
A draw, or tie to Americans, would be a first. But it is not implausible, given that there are 12 swing states up for grabs. As each state has a different number of electoral votes, there are permutations that could have election watchers on tenterhooks. The two candidates could end up all square if Obama retained only New Hampshire, Ohio and Wisconsin out of those 12 states. But give him one more state and he wins. Another map that produces a draw would see the president win Colorado, Iowa, New Hampshire and Virginia.
Tad Devine, a veteran Democratic consultant, told the Politico website: "I think 269-269 could happen. That's not a remote possibility, it's a real possibility."
Most states operate a system by which the winner of a state takes all its electoral-college votes. The electors then meet in December formally to appoint the president. However, the allocation of electors may not be clear for days if one or more races are close. Both parties have small armies of lawyers at the ready in case of disputed ballots, problems with vote counting, or claims of voter fraud or exclusion.
A further complication is that two of the 50 states, Nebraska and Maine, do not operate a winner-takes-all system; they allocate electors according to results in congressional districts and could therefore, in effect, operate a casting vote in the college, which has 538 electors. Experts think, in the event of a draw, Democrats would think it unrealistic to force Biden on Romney, and would allow the Republican to take his running mate, Paul Ryan, as vice-president. Whichever candidate lost the popular vote could also come under moral pressure to concede.
Another controversial, and perhaps more likely, outcome is that Romney wins the popular vote but loses the electoral college, because of Obama's strength in swing states. This would be a repeat of 2000, when Al Gore won the popular vote but lost the electoral college by five votes to George W Bush after a recount in Florida was halted.
With negotiations on balancing the federal budget supposed to be completed by the end of the year, that is a prospect that fills many with dread.