By Steve Schifferes
BBC News, Washington
Senator Barack Obama won the election by gaining an extraordinary level of support among young people, African-Americans and new voters, exit polls show.
Senator John McCain maintained his lead among older voters and evangelicals, but his lead among men evaporated.
The figures from the National Election exit poll, based on interviews with voters at polling stations around the country, show that Mr Obama succeeded in mobilising his key supporters.
And they suggest that the economy was overwhelmingly the most important issue for voters.
Mr Obama had an unprecedented level of support among young people and new voters in the 2008 election.
He won the votes of those under 30 by an impressive 66% to 31%, much higher than in any previous election.
He also has a huge majority of those who voted for the first time, who supported him by 68% to 31%.
This compares to just a slight advantage that the Democrats had in this group in 2004, when John Kerry won new voters by 53% to 46%.
Mr McCain retained the Republican advantage among older voters but lost among the middle-aged, who had supported President George W Bush in 2004.
Mr Obama has succeeded in mobilising African-American voters to his cause to an unprecedented extent, although they were already strongly Democratic.
He won 95% of the black vote, compared to just 4% for Mr McCain.
And he has also built up a big advantage among Hispanic voters, whom Mr Bush partly succeeded in winning over in 2004.
The Democrats led 66% to 31% among these voters, their best-ever result, compared to a 60-40 split in 2004.
Mr McCain led slightly among white voters, by 55% to 43%, but Mr Obama cut the Republican lead among this group compared with the 2004 election.
Barack Obama has made a strong showing among women, exceeding the normal Democratic advantage, while fighting a virtually even battle among men, who went heavily Republican in 2004.
Mr Obama won 56% of the female vote, compared with 51% of women who voted for John Kerry last time.
And he was essentially tied among men, erasing the 55% to 45% advantage that President Bush enjoyed in 2004.
Larry Sabato, political science professor at the University of Virginia, says that the Democrat's appeal to women has been one of the most important keys to his success.
THE PALIN EFFECT
One group that Mr McCain has held on to are evangelical Christians, who make up about one quarter of the electorate.
This group voted by three-to-one for the Republicans despite attempts by Mr Obama to reach out to faith groups - little changed from the previous two elections.
Larry Sabato adds that Sarah Palin's nomination as vice-presidential candidate helped secure this group for Mr McCain.
However, she may have been a negative for his campaign as a whole.
Over 60% of those surveyed said she was not qualified to be president if necessary, compared with 38% who said she was.
IT'S THE ECONOMY
The exit polls suggest that the economy was by far the dominant concern in the election, with 62% citing it as the most important issue facing the US.
Only 10% cited Iraq and 9% terrorism, two issues that dominated the headlines one year ago, while 9% cited healthcare.
Asked what what they thought of the state of the economy, those who said it was in the direst straits were also the strongest backers of Mr Obama.
Those "very worried" by economic conditions voted 59% to 38% for Barack Obama.
Larry Sabato says that, barring other factors, a struggling economy has been a strong predictor of success for the party that is out of power.
The enthusiasm gap
Perhaps the biggest factor in deciding the election was the enthusiasm gap - the difference between the intensity of feeling between McCain and Obama supporters.
One-third of the Obama voters were "excited" by the prospect of his victory, as opposed to just 14% of McCain voters.
That enthusiasm translated into more personal contacts, with twice as many Obama supporters having been contacted in person as McCain supporters.
And 80% of those contacted did in the end vote for the Democrat, suggesting targeting paid off.
The National Election Exit Poll is a sample of 17,856 voters surveyed on election day after they have left the polling booth on behalf of AP and the major US TV networks.