SCENARIOS: How the U.S. election could end up a tie

(Reuters) - The U.S. House of Representatives could decide the presidential election if Republican John McCain and Democrat Barack Obama end up in a tie by carrying a certain combination of states.

The November 4 election is determined on a state-by-state basis rather than a nationwide popular vote. Each state, along with the District of Columbia, is allotted a number of votes in the Electoral College that correspond to the number of representatives it has in the House and the Senate.

Two states, Maine and Nebraska, do not allocate all their electoral votes on a winner-take-all basis -- a fact that could determine the election if the other states tie. The McCain campaign hopes to win one electoral vote in the rural northern half of Maine, while the Obama campaign aims to peel off the Omaha area from the rest of Nebraska, which is solidly Republican.

Most opinion polls show Obama widening his lead in battleground states, making a tie unlikely at this point.

Nevertheless, following are scenarios which could give each candidate 269 electoral votes, one short of the 270 needed to win.

States not mentioned in the following scenarios are assumed to have voted for the same party that they did in the 2004 race.

- McCain wins New Hampshire but loses Colorado, New Mexico and Iowa to Obama. This is the most likely scenario as opinion polls show McCain and Obama are close in these states.

- McCain loses Nevada, Iowa and New Mexico to Obama.

- McCain loses West Virginia, Iowa and New Mexico to Obama. This is less likely as opinion polls show McCain ahead in West Virginia by a larger margin than in Nevada.

- McCain wins Michigan, but loses Virginia, Colorado, New Mexico and Iowa to Obama. This scenario is seen as unlikely because opinion polls currently show Obama holding a substantial lead in Michigan and McCain has given up campaigning there.

- McCain wins New Hampshire but loses Iowa, Nevada and Colorado.

(Compiled by Andy Sullivan)