As anyone not living under a rock — or even under a rock with good Internet access — knows, the presidential race between Sens. Obama and McCain is tied.

One possible 269-269 scenario.

Various polls show one or the other with a slim lead in popular or electoral votes, but the margin of the lead is almost always less than the margin of error. National polls change daily. As of this writing, Pollster.com projects the electoral votes as 202 for Obama, 208 for McCain and 128 as toss-ups, while realclearpolitics.com has 219 for Obama, 189 for McCain and 130 as toss-ups.

A candidate needs 270 out of 538 total electoral votes to win. There are some scenarios (if Obama wins Michigan, Pennsylvania, Colorado and New Mexico and McCain takes Ohio, Indiana, New Hampshire and Virginia, for example) where each man ends with 269 votes, resulting in a tie.

What happens then?

In a tie, following the 12th Amendment, the vote proceeds to the House of Representatives, where Democrats have a majority. However, the amendment stipulates that the voting is done by state, not by representative. So California, with 53 representatives, gets one vote. Wyoming, with one representative, also gets one vote.

According to Robert Longley, the newly elected representatives would break the tie in the electoral college. In the House, the Democrats have a majority of representatives in 27 states, including some traditionally red states like Indiana, Mississippi and the Dakotas. Two states, Arizona and Kansas, are split, and the Republicans have a majority in 21 states. If the state delegations vote by simple majority, Obama would win the tiebreaker.

Some of the majorities are quite slim, (5-4 in Indiana, 7-6 in New Jersey and North Carolina and 1-0 in South Dakota and Vermont), and it would only take a few losses by incumbent Democrats or a few defections to throw the election in the House to McCain.

It is up to each state to figure out how it will determine its vote. Suppose the representatives decided that instead of voting along party lines, they were going to follow the Electoral College and vote for whichever candidate had won their state. In the model used above, where Obama takes Colorado and New Mexico to get to 269, Obama would have carried 22 states, while McCain would have carried 28 states. If the representatives choose to follow the direction of their state’s electors, then the election would go to McCain in the House.

Even if only two states with narrow Democratic majorities in the House decided to switch their votes, it could change the outcome or even throw the voting in the House into a tie.

If that happens, they keep casting ballots until they choose someone. Otherwise, the vice president takes over. In this case, that would be Dick Cheney (start scary music now).

Of course, none of this may come to pass. If it did, it wouldn’t be the first time an election has been tied or disputed.

America’s first tie came in the election of 1800. Thomas Jefferson, the vice president, was running against the incumbent, President John Adams. Aaron Burr and John Jay were also in the race. In those days, the electors cast two votes for president. The candidate with the most votes was president; the one with the second most became vice president.

Jefferson’s party, the Democratic-Republicans, supported him for president and Aaron Burr for VP. The plan was to have one elector cast a vote for Jefferson but not for Burr, giving Jefferson enough votes to win and Burr one less vote, making him VP. The plan only half worked.

Jefferson received 73 electoral votes, enough to defeat Adams for the presidency. By mistake, Burr also received 73 votes, resulting in a tie and sending the election to the House, which was controlled by the opposing party, the Federalists.

Anxious to keep Jefferson out of power, the Federalists cast their votes for Burr. The states controlled by the Democratic Republicans cast their votes for Jefferson, resulting in a weeklong battle, with neither able to capture a majority.

Finally, Alexander Hamilton, a longtime rival of Jefferson and Burr, decided that while he opposed Jefferson, Burr would be worse. On the 36th ballot, he used his influence to get Jefferson the majority. This eventually led to his duel and death at the hands of Aaron Burr.

The disputed election of 1800 led to the duel between Alexander Hamilton and Aaron Burr, pictured above. Most observers feel that an Obama-McCain duel is not a very likely outcome of this election.

In 1804 the 12th Amendment was passed requiring voters to vote separately for president and vice president, alleviating the cause of the debacle of 1800.

In 1824, after the demise of the Federalist Party, there were four viable candidates for president: John Quincy Adams, Andrew Jackson, Henry Clay and William Crawford, all of whom ran as Democratic-Republicans. The votes in the Electoral College were split among the candidates. Jackson received the most votes, but he did not have a majority, so the election went to the House for the only time since the 12th Amendment. Jackson expected to be elected president because he had won the most votes, but Henry Clay detested Jackson and maneuvered for his defeat. Clay threw his support behind John Quincy Adams, giving him the margin he needed to defeat Jackson. Clay was then appointed secretary of state, enraging Jackson supporters.

Jackson won his revenge four years later, in 1828, by winning the presidency and serving two disastrous terms.

In 1876, Rutherford B. Hayes, the Republican, faced Samuel Tilden, the Democrat. Tilden won the popular vote and had 184 electoral votes, needing only one more to win. Hayes had 165 votes, but the 20 electoral votes of Louisiana, South Carolina and Florida (sound familiar?) were in dispute. After a bitter dispute, Hayes was awarded the electoral votes of all three states, bringing him to 185 and edging out Tilden by one electoral vote. It is widely speculated that Hayes had agreed to end Reconstruction in exchange for the support of the southern states.

Most readers remember the election of 2000, where an incredibly close election in Florida gave the electoral victory to Bush by a margin of 271-266 despite Gore winning the popular vote. Although it and the election of 1876 were not ties in the electoral college, they were certainly both close and disputed.

Whatever happens in 2008, voters should realize that there is precedent for it in American history. Elections have been tied before, in 1800 and 1824, and elections have been disputed before, in 1876 and 2000. The winner of the popular vote usually wins, but not always. The election could come down to a few hundred votes in Colorado or Virginia.

This makes it imperative that every single American gets out and votes. As long as they’re in a battleground state, anyway. And as long as they agree with me, of course. If they’re voting for the other guy, they should just stay home.