The odds are that the victor of the American presidential election on 4 November will secure a clean-cut victory in the electoral college - an absolute majority of 270 votes or more.

But that isn't a guaranteed result.

The closer the election gets - and right now the polls have it very tight indeed - the greater the chance that a tie of 269 to 269 will occur.

Nancy Pelosi: The next president of the United States?

There are dozens of ways for the College to tie, given the 51 moving parts: the 50 states (each state assigned a number of electoral votes equal to the total of its House and Senate delegation) and the District of Columbia, assigned three electoral votes by constitutional amendment.

However, most of the ties are nonsensical, since most states have an established tilt to one party or the other. California will not go Republican, and Texas will not vote for the Democrat.

Yet take a look at the current electoral college map, which is based both on historical trends and this month's polls:

The totally safe and likely states for Obama have 200 electoral votes. For McCain, the similar total is 174.

Leaning to Obama at present are Iowa, New Mexico, and Wisconsin, bringing the Democrat to 222. McCain has a slight edge for now in Florida, Missouri, and North Carolina, which would push him up to 227.

That leaves in the toss-up category seven states with a total of 89 votes: Colorado, Michigan, Nevada, New Hampshire, Ohio, Pennsylvania, and Virginia.

How it might happen

Given the results in 2000 and 2004, when Michigan and Pennsylvania voted Democratic, it is not unreasonable to assign them to Obama, bringing him to 260, just 10 votes short of the White House. While Obama has a real chance to win in Colorado, Ohio, and Virginia, all three states have tended to vote Republican in many or most election years.

So what happens if a tie occurs? In two words, a mess

If McCain wins them, while Obama carries two small toss-up states, where recent surveys have given him a tiny edge, Nevada and New Hampshire, the 269-269 tie results.

Alternatively, let's suppose McCain manages to carry New Hampshire - a Kerry state in 2004, but one that has given McCain key primary victories in 2000 and 2008. In this case, if McCain also wins Nevada, and Obama wins Colorado (while our other assumptions remain the same) we again end up with the 269-269 result.

If you mix and match states on the map, in fact, you will quickly see that it relatively easy to produce a tie in the electoral college. So what happens if one occurs? In two words: a mess.

Under the constitution, the election for president is thrown into the US House of Representatives, while the Senate picks the next vice-president (the Senate's presiding officer).

The 'unit rule'

But while the Senate simply requires a majority of its 100 members to select the vice- president, the House must vote by states, with each state delegation having a single vote, and a majority of the states (at least 26 of 50) required to agree on the winner.

Thomas Jefferson: The unit rule was fairer in 1800 than it is today

This is called "the unit rule". The founding fathers centred the idea on the fact that the nation was a confederation of states rather than a pure democracy of individual voters. Just as the electoral college is state-based, the House selection of the president in the case of deadlock revolves around the states.

The unit rule has been employed twice in US history, in the long-ago elections of 1800, when Thomas Jefferson emerged as president, and 1824, when John Quincy Adams was elected.

Both elections were controversial, but not mainly because of the unit rule. Two centuries ago, the unit rule was less undemocratic since population disparities among the handful of states were smaller, and Americans were much more accepting of elite control and a lack of popular sovereignty.

The United States of 2008 has very different values.

Think about what House selection of a president would mean today. Gargantuan California would have the same single House vote in choosing the new president as sparsely populated Wyoming, even though California has about 70 times its population. The votes of the mega-states of Florida, New York, and Texas could be cancelled out by the tiny populations of Montana, Rhode Island, and South Dakota.

Furthermore, large state delegations could internally deadlock via tie votes. Some large states might be deprived even of their single vote for the presidency. Tens of millions of people could be disenfranchised in this fashion.

Equal pigs

Meanwhile, all the small states with single House members will certainly be counted. The smaller the House delegation, the more likely the state's House members will be able to reach agreement or at least finish their tally. All pigs would be equal, but in this odd Orwellian case, the tiny pigs would be more equal than the huge ones.

How could a president elected in this fashion govern effectively?

"The turmoil would be an international embarrassment, at the very least"

If the public reaction to the Supreme Court's Bush v Gore decision in 2000 was bitter, one can scarcely imagine the outrage that would greet a modern application of the unit rule. By the way, well over 90% of the American people are unaware of this constitutional provision.

So if a 269-to-269 tie occurs, what is the likely outcome?

On the surface, one would think Barack Obama would be likely to capture the Presidency - and that would be my early bet. Already, Democrats control a majority of 27 states' House delegations, and the party is expected to pick up additional seats this November - perhaps pushing a couple of other state delegations their way.

But it may not be so straightforward.

The new Congress would take up the matter quickly in early January, but would have to produce a result by Inauguration Day (20 January).

Suppose some large state delegations reach an impasse? Suppose some of the more conservative "Blue Dog" Democrats decide to break with their party and vote the way their districts or states did, presumably for John McCain?

The turmoil would be an international embarrassment, at the very least. If a stalemate results, the constitution provides that the vice-president-elect would become acting president until the gridlock is broken. Presumably, that would be Joe Biden since Democrats are expected to have a comfortable majority in the new Senate.

However, what if the Senate gets tied up in parliamentary knots? It's been known to happen.

Then America is likely to get its first female President, at least until the deadlock is broken. No, she won't be named Hillary Clinton, nor will the acting chief be Sarah Palin. Next-in-line will be the Speaker of the House, Democrat Nancy Pelosi.

Professor Larry J Sabato is director of the University of Virginia's Center for Politics and the author of A More Perfect Constitution.

http://news.bbc.co.uk/2/hi/americas/7626471.stm