For weeks President Barack Obama and Republican challenger Mitt Romney have battled it out for the handful of swing states that will decide the November election. But there's another possibility to entertain - a draw.

Here's how it could happen and what would follow.

How could a draw even come about?

A draw does not mean both candidates win exactly the same number of votes (in 2008, 131.4 million Americans cast ballots), because the US presidential election is really a contest for states, not individual votes.

Under an 18th Century system called the electoral college, each state has a number of electoral votes in proportion to its population. On election day, voters are not actually selecting a presidential candidate, but from among slates of officials called electors who pledged to choose a candidate at a later ballot electoral college.

With two exceptions, states award their electoral votes to the candidate that wins the most votes in the state. Nebraska and Maine each award two electors to the winner of the state popular vote and split their remaining electoral votes by congressional district.

The goal of the presidential election is to win 270 electoral votes, a majority of the total. When strategists talk about a candidate's path to victory, they have in mind the right combination of safe states - like California for the Democrats and Texas for the Republicans - and swing states such as Ohio and Pennsylvania, to achieve to that total.

This year, the electoral map holds a few far-flung scenarios under which Mr Romney and Mr Obama could bag the precise combination of states to come away with 269 votes each.

For instance, if Barack Obama wins the key swing states Ohio, New Hampshire and Wisconsin - but Mitt Romney takes all the other swing states including Nevada - both would finish on 269 electoral college votes each, a tie.

Then what happens?

If the 6 November election is close enough to result in an apparent electoral college tie, expect weeks of nasty legal fights, vote recounts and public protests.

If the result ultimately holds, on 17 December the electors convene in 50 states and Washington DC to cast their votes for president according to wishes of voters in their states.

This vote formally ratifies the result of the 6 November election.

According to procedures for a draw codified in the 12th Amendment to the US constitution and fleshed out in statute, the 435-member House of Representatives meets in January after the new Congress is sworn in.

Then, they pick the president.

How will the House vote?

Each state gets one vote in the House election, with the party in control of a state's delegation choosing how it votes.

The House is currently dominated by the Republican party. The House election would take place after the new Congress has been seated, and analysts predict the Republicans will maintain control of more state delegations than the Democrats. Mitt Romney will almost certainly win the presidency in an electoral college tie.

Who is chosen as vice-president?

The 12th Amendment gives the choice of vice-president to the Senate, which meets the same day as the House, down the hall in the US Capitol building.

If as expected the Democrats retain a narrow majority, they will select Vice-president Joe Biden to stay in that job for the next four years. But if Republicans eke out control of the chamber in the 6 November election, they will choose Mr Romney's running mate Congressman Paul Ryan of Wisconsin.