The US Constitution requires that the President and Vice President be elected by a majority vote of the Electoral College. Because the Electoral College has 538 members, a majority would equal 270. However, it is possible to have a 269-269 tie. Here are some ways in which a tie in the Electoral College could happen - and how it would be broken.
How a tie could happen
The states Kerry won in 2004 account for 252 electoral votes. We need 18 more votes to win outright, 17 more to tie. Here are just a few ways in which a tie mighty happen.
1. We win all of the states Kerry win, plus Iowa (7 electoral votes) - which Bush won by just a few thousand votes - and either Arizona (10) or both Nevada (5) and New Mexico (5).
2. We lose Wisconsin (10) but win in either Florida (27) or both Ohio (20) and Iowa.
3. We win Colorado (9), New Mexico, and Iowa, but lose New Hampshire.
4. We lose Pennsylvania (21) but win Florida and Missouri (11).
5. We lose Michigan (17) but win Florida and Iowa.
6. We lose Michigan but win Ohio, New Mexico, and Colorado.
7. We lose Michigan, New Hampshire, and Pennsylvania but win Florida, Iowa, Ohio, and either New Mexico or Nevada.
Casting and counting the electoral votes
So as you can see, there are a number of ways in which the two tickets can deadlock at 269 EVs apiece, many of which I haven't even mentioned. So what happens then? Some of you know exactly how the process works, but for those who don't, here's a synopsis.
1. November 4, 2008: When voters vote in presidential elections, they are actually voting for a slate of electors (usually nominated by state parties) who are pledged to vote for a specific Pres/VP ticket.
2. December 15, 2008: Those electors will gather in their state capitals to officially vote for the new President and VP on the Monday following the second Wednesday in December. In some states (but not all), it is entirely possible that one elector will vote for the candidates opposite the ones for whom they were pledged to vote, and thus break the tie.
3. January 6, 2009, or thereabouts: Shortly after the 111th Congress convenes, Congress will meet in joint session to officially count the electoral votes. If a candidate receives at least 270 electoral votes, they are elected.
4. If no candidate receives 270, however, Congress settles the matter. Amendment XII tells us how that would work.
...the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice. ...In other words, each state's House delegation gets one vote to determine who will become the next President of the United States, while each Senator votes for the next Vice President. This means that one can assume that a state's vote for President will go to the candidate whose party has the majority of Congress(wo)men from that state, while each Senator will vote alonmg party lines.
The person having the greatest number of votes as Vice-President, shall be the Vice-President, if such number be a majority of the whole number of Electors appointed, and if no person have a majority, then from the two highest numbers on the list, the Senate shall choose the Vice-President; ... and a majority of the whole number shall be necessary to a choice.
What this means for 2008/2009
So had the Electoral College deadlocked at 269 apiece in the 2004 election, Bush would still have been declared the winner, receiving the votes of 28 states to Kerry's 19 (assuming Vermont's lone rep, then Independent Bernie Sanders, would've voted for Kerry). However, thanks to the gains Democrats made in 2006, we now hold a 26-21 lead in delegations, with Arizona, Kansas, and Mississippi tied. (Don't ask me how those three states would vote.)
What this tells us is that providing the Democrats do not lose any seats in the House - or that any seats they do lose are in states where it wouldn't cost them the majority in any state's delegation - then we can likely expect a Democratic President in 85 weeks.
Many expect the Democrats to expand their lead in the Senate, winning at least the seat in Colorado, if not New Hampshire, Virginia, or elsewhere. Let's hope this happens, but if there's anything 2005 and 2006 taught us, it's that things can change quite a lot in a short time. (A one-seat majority is not much of a majority!)
Now imagine split control of Congress. Say the Democrats control a majority of delegations in the 111th House, while the GOP wins back the Senate. If they vote on party lines, this would result in a bipartisan split - a Democratic President and a Repub VP - the first time since 1801 that the two have been of opposite parties.
Then there are so many other intriguing possibilities. Ordinarily, a majority in the Senate equals 51 votes, but right now there are only 99 Senators because of the passing of Senator Thomas, so 50 is a majority. In Maine and Nebraska, the winner in each Congressional district gets one vote and the winner of the state as a whole gets two. Then there's the issue of faithless electors - which I barely touched on here.
Confused? I don't blame you. If I had my way, the Electoral College wouldn't exist. But since it doesn't appear to be going away anytime soon, I hope this gives you a better sense of what would happen in the event of a tied Electoral Vote.
Of course, I also hope that a landslide Democratic win will allow us not to have to worry about that.