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Under the US Electoral College system that decides the presidency, there's a real chance the candidates could tie. Here's what would happen next. It's not pretty.
On November 6, Time magazine writes, there's a small chance that the presidential election will deliver Obama 269 electoral college votes, and the Romney the same total.
Under the US system, each state is allocated a set number of "Electoral College" votes, on a winner-takes-all basis, pegged to population. So if Obama wins a majority of the vote in populous California, for example, he would win its 54 Electoral College votes; if Romney wins Texas he will get its 34 votes and so on. (See a full list of states' votes here.)
It would help matters if the Electoral College (which dates from 18th century, with only minor tweaks) had an odd-number total of votes. But, no, states combine to 538 - meaning a 269-269 tie is possible (and has happened, with a smaller but still even-numbed College, in 1796 and 1800).
ABOVE: A Politico.com chart illustrates how how Obama can be slightly behind in nationwide polls, but still hold an edge in the Electoral College that will decide the race - thanks to his lead in a majority of the so-called battlegrounds - the 16 states that are neither safe Democrat or safe Republican. 270 Electoral College votes are need to win the presidency. The bigger the state, the bigger its number of Electoral College votes.
We know from daily tracking polls, that the race is very close.
One poll-of-polls (which could change by the time you click on the link) gives Romney a 1% lead.
But with most states solidly Republican or solidly Democrat, the race comes down to 16 swing states.
And here, the Electoral College maths favours Obama - who has more so-called paths to victoray (combinations of states he could win that would give him a 270 Electoral College vote majority).
Pundits say it's hard to see Romney getting to the magic unless he wins the two largest of the 16 swing states: Florida (29 Electoral College votes) and Ohio (18), as Obama holds a lead in most of the rest.
The President has maintained a small but stubborn lead in Ohio; Romney has been a nose ahead in Florida.
With the race so close, there is a small but real possibility of a 269-269 Electoral College tie on November 6.
Time has detailed what would follow.
In short, only around half of states require their Electoral College delegates to actually follow the popular vote in their state in when they go to Washington DC to actually cast their (always seen as largely ceremonial) vote.
The presumption is that none of the Electoral College members would want to go against the majority vote.
Assuming that's the case, under a 225-year old rule, the lower house of Congress would decide the presidency. The formula is complex, but it boils down to favouring the majority - which, barring a miracle, will remain the Republicans after November 6. They would appoint Mr Romney President.
But here's the twist: the Senate gets to pick the Vice President. The race for the Senate (where one third of seats are up for grabs this year) is tighter, but the Democrats are expected to maintain their majority. The Democrats would appoint Joseph Biden Vice President.
I'm not sure what consitutional experts would make of a Romney-Biden presidency, but iw would definitely keep the late night talk show hosts in material for the next four years.