WASHINGTON - IT'S a nightmare scenario for Democrats - their nominee Barack Obama winning the popular vote while Republican John McCain ekes out an Electoral College victory in America's state-by-state process of selecting a president.

Sure, Mr McCain trails in every recent national poll.

Sure, surveys show that Mr Obama leads in the race to reach the requisite 270 electoral votes to win the presidency.

Sure, chances of Republicans retaining the White House are remote.

But some last-minute state polls show the Republican nominee closing the gap in key states - Republican turf of Virginia, Florida and Ohio among them, and Democratic-leaning Pennsylvania, too.

To win, a candidate must win at least 270 of the 538 electoral votes distributed to states roughly in proportion to their population. In most cases, the candidate who wins a plurality of votes in a state wins all of that state's electoral votes.

If the tightening polls are correct and undecided voters in those states break Mr McCain's way - both big ifs - that could make for a repeat of the 2000 heartbreaker for Democrats that gave Republicans the White House.

In 2000, Democrat Al Gore narrowly won the popular vote by 537,179 votes. But Mr George W. Bush won the state-by-state electoral balloting that determines the presidency, 271 to 266.

The outcome wasn't clear until a 36-day recount awarded Florida, then worth 25 electoral votes, to Mr Bush by just a 537-vote margin.

Before the 2000 election, political insiders had speculated just the opposite, that perhaps Bush would win the popular vote but lose the presidency to Mr Gore.

One day before the 2008 election, Mr Obama sat atop every national poll.

Enthusiastic by all measures, the Illinois senator's Democratic base was expected to run up the score in liberal bastions of party strongholds such as New York and California.

But the race appeared to be naturally tightening in top battlegrounds that each candidate likely will need to help them reach the magic number in the Electoral College, electoral-rich Florida, Ohio, Pennsylvania and Virginia among them.

To win, Mr McCain must hold on to most states that went to Bush in 2004, or pick up one or more that went to Democrat John Kerry four years ago to make up for any losses.

Mr McCain's biggest target for a pickup is Pennsylvania, which offers 21 votes and where several public polls show Mr Obama's lead shrinking from double digits to single digits.

Mr McCain faces a steep hurdle. Mr Obama leads or is tied in a dozen or so Bush-won states, and has the advantage in most Kerry-won states.

The Republican's campaign argues that as national surveys tighten, Mr McCain's standing in key states also rises and that, combined with get-out-the-vote efforts, will lift Mr McCain to victory in Bush states and, perhaps, others.

'What we're in for is a slam-bang finish. ... He's been counted out before and won these kinds of states, and we're in the process of winning them right now,' Mr Rick Davis, Mr McCain's campaign manager, said on Sunday.

Mr Obama's team is awash in confidence.

'We think we have a decisive edge right now' in states Mr Bush won four years ago, said Mr David Plouffe, Mr Obama's campaign manager.

There's still another possibility, perhaps more improbable than the first - that Mr McCain wins the popular vote while Obama clinches the White House.

True, Democrats have been fired up all year.

True, Republicans haven't been.

True, Mr Obama and Mr McCain have been faring about even among independent voters.

But there are signs that the Republicans' conservative base has rallied in the final stretch and these voters usually turn out in droves, even if lukewarm on the candidate.

Then there's the question of a tie in the Electoral College. In that case, members of the next House would select the winner.

If Mr Obama carries every state that Democrat John Kerry won in 2004, plus Iowa, New Mexico and Nevada, then he and Mr McCain each would have 269 electoral votes.

A tie also would result if Mr McCain takes New Hampshire from the Democrats' column but loses Iowa, New Mexico and another state that Mr Bush won, Colorado.

In an election year that's defied conventional wisdom time and again, anything can happen. -- AP