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By Steve Schifferes
BBC News, Washington
Many more people are voting early, which could indicate increased turnout
The key to victory in the US presidential election will be determined by which side gets out the vote.
That is because only a little more than half of eligible voters actually turn up at the polls - a percentage that only rose modestly despite the keenly contested presidential election in 2004.
In the 2004 presidential election, the Republicans won the battle by mobilising more of their supporters than the Democrats.
See turnout in US presidential elections since 1960
This time, the Obama campaign is determined to get out its vote and is pulling out all the stops.
The McCain campaign is hoping that the nomination of Sarah Palin will help mobilise the Christian conservatives who were so important in 2004.
The Ground War
The Democrats have heavily invested in the "ground war" in key states, setting up local campaign offices in every county in states like Ohio, Florida and Virginia since the end of the primary season.
Barack Obama has generated enthusiasm among young people
Other groups supporting the Democrats, such as trade unions, are also spending millions in order to get out their vote; funding telephone and door-to-door canvassing and targeted advertising campaigns.
They are also adopting Republican tactics by targeting different groups of voters with different messages through tactics such as direct mail and targeted video messages.
And there is already some evidence that this year the Democrats are reaching more of their supporters than the Republicans.
According to Professor Sunshine Hillygus of Harvard University, the Democrats are outpacing the Republicans in both traditional and new methods of outreach.
John McCain is hoping Sarah Palin will help mobilise evangelical Christians
But she says that, with the economy such a dominant issue, the Republicans are having more difficulty in targeting their supporters with "dog whistle issues" that are only mentioned below the surface of the national campaign, such as abortion.
Recent polling by the Democracy Corps suggests that three times as many people have been contacted door-to-door by Democrats than Republicans (21% to 6%), one-third more have received phone calls (35% to 24%) and twice as many have received campaign emails (26% to 12%).
In addition, one quarter of Democrats have watched campaign commercials or videos online, compared to one in eight Republicans.
Differential turnout
The Democrats need to increase their turnout because many of their key supporters are generally less likely to vote.
There are big differences in turnout among key groups in US - with the better educated, richer and older people generally turning out in greater numbers.
For example, Mr Obama is running strongly among young people, but in the last presidential election only 46% of 18-24 year-olds voted, compared to 72% of those over 55.
So the under-35s made up 30% of the potential electorate, but only 25% of actual voters, while the over-55s were 31% of the electorate, but 35% of actual voters.
Similarly, white voters were more likely to turn-out to vote, with 67% voting. Only 60% of blacks went to the polls, 47% of Hispanics, and 44% of Asians.
And people on lower incomes are also much less likely to vote, with only 46% of those near the poverty line with family income of $20,000 or less turning out, compared to 81% of those earning over $100,000 per year.
Mr Obama does enjoy one advantage - he is running ahead in the polls among college-educated voters, who are one of the groups who are much more likely to vote.
In fact the differential voting effect of education is even steeper than income or age. So, for example, young people in college are almost as likely to vote as others with a college education.
One of the leading experts in turnout, Curtis Gans of the Center for the Study of the American Electorate, says that "this year, higher turnout will almost certainly benefit Obama and the Democrats."
However, he says that turnout among young people is unlikely to rise substantially.
Early voting
This year the Democrats are making a big effort to encourage people to vote early, and with 32 states allowing it, estimates suggest that up to 30% of voters will have cast their ballots before election day, up from 22% last time.
Millions of new voters have registered across the US in the run-up to the vote
The latest Pew opinion poll, carried out in the last full week of campaigning, suggests that the Democrats enjoyed a 19% lead (53% to 34%) among early voters.
Many early voters were hoping to avoid the problems of long queues and potential chaos at polling booths in swing states.
The evidence also suggests that the Democrats have also succeeded in the drive to register more of their supporters this year, with new registrations running more than two to one Democratic in states where party affiliation is recorded.
This is just as important as election day get-out-the-vote efforts. While around 15% of registered voters fail to get to the polling booth, nearly of 25% of eligible citizens are not even registered to vote, according to the US Census Bureau.
And there is a similar difference in voter registration. For example, only 52% of people without a high school education were registered to vote, while 82% of college-educated had registered.".
Groet, Jonas