By Greg Giroux, CQ Staff

It’s a nightmare scenario: The votes are counted on Election Night four weeks from now, and Barack Obama and John McCain have each secured 269 electoral votes, one fewer than the majority the Constitution requires to be elected president — and throwing the election to the House of Representatives to decide.

It’s a distant possibility, to be sure, and became only more so with polls last week showing Obama gaining ground in the race — even in battleground states like Ohio and Florida — probably as a result of the Wall Street crisis. Only one presidential election has ever yielded an Electoral College tie: the 1800 race in which Thomas Jefferson and his Democratic-Republican running mate, Aaron Burr, each had 73 votes and the House eventually picked Jefferson; eight years later the 12th Amendment was ratified mandating separate votes by electors for president and vice president. And in the last two weeks the momentum has clearly swung Obama’s way.

Recipe for a Stalemate: Click Here to View Chart

But given the continued volatility of the electorate and the closeness of the race — which follows the two squeakers won by George W. Bush , with 271 electoral votes eight years ago and 286 last time — a tie is not entirely implausible.

“It would be an unusual, maybe even startling event, but I don’t think it’s one that we could discount entirely,” said Robert W. Bennett, a law professor at Northwestern University who has written about the Electoral College.

Using the 2004 map as a template — and most of the states will vote for the same party as they did then, given the basic political demographics — a tie would result this way: Obama would pick up Colorado (nine electoral votes), Iowa (seven) and either New Mexico or Nevada (five votes each). All of those are states that Bush carried four years ago, but where Obama is running even or ahead of McCain in the current polls. For his part, McCain would then have to pick up New Hampshire (four electoral votes), which Democrat John Kerry carried in 2004. McCain won the New Hampshire primary both in January and eight years ago and is polling well there at the moment.

This analysis assumes that Nebraska and Maine, which give one electoral vote to the winner of each congressional district and two electoral votes to the statewide popular vote winner, will continue their longstanding pattern of not splitting their electoral votes between candidates.

Another potential complication is the operation of the Electoral College itself. The electoral vote count after the polls close Nov. 4 will technically be nothing more than a projection; the Democratic and Republican electors will actually cast their votes six weeks later, on Dec. 15, in what should be a pro forma affair in their state capitals.

Among the 538 electors, there could well be one or more renegades who switch sides, even though electors are almost all party activists. Only half the states have laws against becoming what’s known as a “faithless elector.” The electoral votes will be counted and announced on Jan. 6 in a joint session of Congress. If it’s revealed that one or more faithless electors has switched sides, the losing party would challenge those electoral votes as not “regularly given.”

“All hell would break loose if there was a faithless vote that seemed to break what would otherwise be a tie,” Bennett said.

If the tally produced no Electoral College majority for either candidate, the House would elect the next president. Each state’s delegation, regardless of its size, would get one vote; it takes a 26-vote majority to win.

This scenario would seem to ensure Obama’s election. Democrats currently hold most of the House seats in 27 states (compared to 21 for the Republicans and two that are split) and are on course to increase their majority in the 111th Congress, which convenes Jan. 3, three days before the electoral votes are counted.

If history is any guide, an Electoral College tie could produce some fierce politicking in Congress. In 1801, it took 36 ballots in the House before Jefferson triumphed over Burr. In 1825, the only other time the House elected a president, it chose John Quincy Adams over Andrew Jackson, even though Jackson had won a plurality of both the electoral and popular votes.