A confident Barack Obama stands ready to make history by becoming the first black president of the United States after wrapping up a marathon two-year campaign.
But Republican John McCain, trailing in most national polls, has promised an upset of an almost equally historic order over the Democratic candidate in Tuesday's election.
Separated by 25 years and a vast political gulf, Obama and McCain have agreed on one thing during the longest and one of the most divisive presidential campaigns ever – a promise to close the door on the era of George W. Bush.
But they are deeply at odds over how to fix the nation's crumbling economy and end the five-and-a-half year war in Iraq – issues that sank Bush's rating to a record low toward the end of his eight-year presidency.
A victory for Obama, whom Republican opponents have characterised as outside the American mainstream, could mark a dramatic shift in what has in the past been a centre-right country, some liberal commentators believe.
Conservatives point out though that Republicans regularly bounce back with equal or greater strength after wins for Democrats.
Record numbers of Americans are expected at polling stations. About 29 million citizens have already voted and by the time polls close, as many as 140 million Americans are estimated to have had their say on who becomes the next leader of the free world.
Europeans, who have over the past few years ushered in centre-right governments, overwhelmingly favour the Democratic candidate. In Berlin, an Obama speech attracted 200,000 people.
International polls have consistently placed Obama above McCain by a four-to-one margin. An informal poll of swissinfo readers showed support for Obama at 76 per cent on Tuesday versus McCain's 24 per cent.
By harnessing the power of the internet, text messaging and viral marketing, Obama mobilised a record fundraising effort and capitalised on a US demographic shift as more young and non-white voters enter the electorate.
Republicans have tried to rein in Obama's surge, calling him inexperienced, too liberal and tainted by associations with the political left to trust with the presidency. The message appealed to core Republican voters, but has not resonated with a significant number of Democrats and independents.
The early vote tally suggested an advantage for Obama, showing that Democrats voted in larger numbers than Republicans in key states.
Democrats also anticipated strengthening their majority in the House of Representatives and in the Senate, although Republicans battled to hold their losses to a minimum, and a significant number of races were rated as toss-ups in the campaign's final hours.
"I'm feeling kind of fired up. I'm feeling like I'm ready to go," Obama told nearly 100,000 people gathered for his final rally on Monday night in Manassas, Virginia. The state has not voted for a Democratic presidential nominee in 44 years but polls suggest it is now tipped toward Obama.
McCain, a 72-year-old four-term Arizona senator and Vietnam War veteran, ended the contest on Monday with a frantic and exhausting dash through several traditionally Republican states still not securely in his camp. Some are even leaning toward his rival.
McCain ended the endurance test after midnight in Arizona, his home state. Obama ran television commercials in Arizona in the campaign's final days after polls showed the race tightening.
By Monday, 47-year-old Obama - a first-term senator from Illinois - was favoured to win all the states Democrats captured in 2004, when Bush defeated Democratic Senator John Kerry. That would give him 251 electoral votes.
He was leading or tied in several states won by Bush, giving him several paths to the 270-vote threshold needed to win. Victories in Ohio or Florida, or in a combination of smaller states, could provide that.
Obama has planned a quick campaign stop in Indiana on Election Day before a massive outdoor rally in his adopted hometown of Chicago.
Despite his lead in the polls, Obama warned against overconfidence. "Even if it rains tomorrow, you can't let that stop you. You've got to wait in line. You've got to vote," he said.
The likelihood of Republican defeats in both the presidential and congressional races was not lost on the current president – Bush has become virtually invisible in the final days of the campaign.
swissinfo with agencies
Swiss monitors eye US elections
Election officials across the US have braced for record turnout and are hoping to avoid problems at the ballot box.
Around 100 observers from the Organization for Security and Cooperation in Europe (OSCE) have been sent to the US to monitor voting.
They include two Swiss parliamentarians, who are monitoring the election in Virginia.
"Our task is to monitor ten to 12 polling stations in Richmond, the capital," said Christian Miescher of the rightwing Swiss People's Party.
Social Democratic parliamentarian Hans Widmer has joined Miescher.
Miescher reports no problems in early elections, but during the past weeks, there have been accusations of voter-registration fraud by conservatives and voter suppression by liberals.
Civil rights groups have said that poll hours need to be extended and voting machines added in some black precincts, but a Virginia judge on Monday threw out a lawsuit by one civil-rights organisation.
Switzerland and the US
There are around 1.2 million Swiss-Americans and Americans with Swiss roots.
Of the close to 74,000 Swiss nationals registered in the United States at the end of 2007, over 52,000 were dual citizens.
Of an estimated six million Americans living abroad, roughly 30,000 are registered in Switzerland.
Votes from Americans living in Switzerland are directed to the state in which they last resided before leaving the country.
Voting rights extend to US citizens even if they do not own property or have ties to the country.
Only sixteen states however allow citizens who have never lived in the country to vote.
Most states send overseas ballots 30-45 days before the election.
Voters must return the ballots before their state's deadline but can pick up an emergency ballot from an embassy, consulate or on the internet.
The world headquarters of American Citizens Abroad, an advocacy organisation, is in Geneva.