A huge surge in early voting across key states appears to be helping the Democratic candidate, Barack Obama.
In one swing state, North Carolina, Democratic early voting is up 400% in the first week, with similar patterns in Ohio, Iowa, Nevada and New Mexico.
Early voting is now taking place in 34 states, although votes are only counted on 4 November.
Up to one-third of all ballots are expected to be cast before election day, compared to one in five in 2004.
In the past, Republicans have tended to benefit from early voting.
"This is like a mirror image of what we've seen in the past," said Paul Gronke, of the Early Voting Information Center at Reed College. "This cannot be good news for John McCain."
Overall, early voting is running about twice as high this year as in 2004 in many states.
In states that register voters by party, the party affiliation of early voters is known - though it is not certain that all registered Democrats will vote for their presidential candidate, and the data does not give information on how independent voters will cast their ballots.
One reason for early voting has been the fear of long waits at the polls on election day - which may be an even bigger problem this year because of higher turnouts.
They only get to vote once
Rich Beeson, political director, Republican National Committee
The Democrats seem to be bringing out early voters in key swing states.
In Clark County, Nevada, which covers the largest city, Las Vegas, Democrats have been outnumbering Republicans in early voting by a two-to-one margin, higher than the 52% majority for the Democratic candidate, John Kerry, in 2004.
In Cuyahoga County, Ohio, which covers the city of Cleveland, 45,000 Democrats have voted early and only 10,000 Republicans, far above the two-to-one margin by which Mr Kerry won in 2004.
In New Mexico, which was won by George W Bush by 6,000 votes in 2004, twice as many Democrats as Republicans have been voting early.
And in North Carolina, which has traditionally been Republican but is now a swing state, of the 480,000 people who have voted early, 54% were Democrats, 27% Republicans, and 16% independents.
Michael McDonald, a voting expert at George Mason University, said the data from North Carolina was stunning.
"North Carolina, in particular, is off the charts," Mr McDonald said. "This is outside of what we expected."
The Republicans have discounted the importance of these early voting figures, saying that they just reflect the most partisan voters who would have voted for their party anyway.
"They only get to vote once," said Rich Beeson, political director for the Republican National Committee.
And not all the early voting trends are favouring the Democrats.
In Florida, absentee ballots are running three to two in favour of the Republicans, while in Colorado early voting is running roughly in line with overall voter registration.
With some evidence that election polls are tightening, the big increase in early voting is making it more difficult for Senator McCain to close the gap on Mr Obama.
In past elections, up to one in five voters have decided in the last weeks of the campaign, and sudden events, such as a foreign policy crisis, could alter the outcome.
"If there's a last minute surge because of some event to the trailing candidate, well, the train has left for an awful lot of people these days," said Andrew Kohut, president of the Pew Research Center, which carries out election polls.
The latest Pew polling suggests that 24% of people are likely to vote early, while 9% said they had already voted.
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