For weeks the incumbent Democrat and his Republican challenger have been neck and neck in the opinion polls and while Mr Obama feels he has inched ahead in a few key states and believes his opponent is reaching desperation point, Mr Romney and his followers insist the momentum is firmly with them.
For all the political rhetoric, a daily tracking poll of 3800 voters yesterday again confirmed it was too close to call with the President on 48% and the former Massachusetts governor on 47%. Underlining the uncertainty, the poll has a margin of error of three points either way. An ABC News snapshot had them tied on 48%.
The nature of the US presidential voting system means that victory and defeat are delivered in just a few swing states. So today, as they have done over the weekend, each candidate will fly from dawn to midnight across the key battlegrounds.
In the last day or so, Mr Obama and Mr Romney have been holding several rallies in each. The President has been to New Hampshire with visits planned for Ohio, Colorado and Florida while the ex-governor has been in Iowa before heading to Ohio as well and Pennsylvania and Virginia. And there will be more last-ditch visits today.
The Obama camp cited planned trips by the Republican candidate to Virginia and Florida today – states the Romney camp believed it had sewn up – as evidence of a last-minute panic by the challenger.
David Axelrod, a key Obama strategist, said: "They understand that they're in deep trouble. They've tried to expand the map because they know in states like Ohio, they're behind and they're not catching up at this point."
Ohio, seen as possibly the most key state, saw a poll placing Mr Obama on 50% to Mr Romney's 48% but, again, the margin of error was plus or minus two points.
There, Mr Romney, running under the slogan "real change on day one", told supporters that he would deliver "a fresh start, a new beginning" in Washington.
"We're Americans. We can do anything," he declared to cheers.
In Iowa, he said: "The only thing that stands between us and some of the best years we can imagine is a lack of leadership – and that's why we have elections."
Earlier in New Hampshire, Mr Romney criticised his opponent for saying that voting would be their "best revenge" on the Republicans. "Vote for revenge? Let me tell you what I'd like to tell you: vote for love of country. It is time we led America to a better place."
Then in Colorado, the Republican challenger told supporters tomorrow's election would be "a moment to look into the future, and imagine what we can do to put the past four years behind us". He added: "We're that close right now. The door to a brighter future is there."
Meanwhile, at his rally in New Hampshire, Mr Obama, whose campaign slogan is "Forward!", assured his faithful that he would work across party lines to break the political gridlock in Washington yet insisted he would not compromise on priorities such as health care and college financial aid. "We have come too far to turn back now," he said.
In Ohio, the President claimed the election was a choice "about two different visions for America – the top down vision that crashed the economy or a future built on a strong and growing middle class".
At a rally over the weekend in Virginia, Bill Clinton, the former Democrat president, gave an address with a croaky delivery, joking that he had "given my voice in the service of my president".
Unlike the elections to Congress, those for the president use an electoral college with states receiving votes in relation to their size. So California has 55 votes while the smallest populated have just three. The magic number which ensures victory is 270.
The result is expected in the early hours of Wednesday morning but some analysts are warning Americans might have to wait weeks for a clear result if there is a photo finish.
In 2000, the US Supreme Court ended up deciding the result when Democrat Al Gore won the popular vote but Republican George W Bush won the electoral college with such a narrow margin in Florida that it sparked a recount and massive legal battle over the infamous 'hanging chads'.
Each candidate is expected to have spent more than $1 billion or around £620 million, a figure which doesn't include hundreds of millions spent by so-called super political action committees.
Meanwhile, Edinburgh University has created computerised Obama and Romney candidates so that people keeping up with the US presidential race can now enjoy a one-to-one chat with virtual candidates. Log onto www.nutbot.net/talking_head/