US President Barack Obama and Republican challenger Mitt Romney remain essentially tied in the race for the White House with razor thin margins in four key swing states, according to a Reuters/Ipsos daily tracking poll released this evening.

Three days before the 6 November election, both men are neck and neck in Ohio, Florida, Virginia and Colorado, the poll showed.

Nationally, the electorate is divided.

Of likely voters polled, 47% said they would back Mr Obama, the Democratic incumbent, while 46% said they would back Mr Romney, the former Massachusetts governor.

The results fall within the polls' credibility interval, a tool used to account for statistical variation in Internet-based polls.

The two men have been locked in a tight race for weeks.

Both are doing final campaign swings over the weekend, trying to sway a small group of remaining undecided voters and to encourage their supporters to get to the polls.

A handful of states that traditionally swing between voting for Republican and Democratic presidential candidates will determine who wins the White House. All of them are close.

In one of the biggest prizes of the election - Ohio - Mr Obama has a very slight lead over Mr Romney with 46% compared to 45% support among likely voters, the poll showed.

In Florida, another big prize, they are tied at 47%.

In Virginia, Mr Obama leads Mr Romney 48% to 45% among likely voters. In Colorado, Mr Romney leads Mr Obama 47% to 45%.

"It's really going to come down to the wire," said Ipsos pollster Julia Clark, adding that Mr Obama still had a better likelihood of winning the 270 state electoral votes needed to secure victory.

However with the data tight in the swing states for both candidates, she added: "I don't think we can count any of them in the bag yet."

"The electoral map does favour Obama," Clark said. "It's just so on a razor's edge."

The precision of the Reuters/Ipsos online polls is measured using a credibility interval, which in the case of the national poll is plus or minus 3.4% for likely voters.

Candidates make final campaign push

In duelling campaign appearances in swing states, both Mr Obama and Mr Romney battled over the economy following the publication of the latest US job figures.

Mr Obama touted his administration's auto industry bailout to a crowd in Ohio, part of the US industrial heartland.

He continued to use the topic to attack his opponent, who he says was opposed to the government-led rescue plan.

Meanwhile Mr Romney scolded Mr Obama for encouraging his supporters to get "revenge" at polling stations, saying he found it troubling.

He told a crowd of supporters in Iowa that he wanted people to instead vote “for love of country”.

Mr Obama made the revenge comment during a stop in Ohio yesterday, where he told them not to boo when they heard Mr Romney’s name.

“Don’t boo, vote. Voting’s the best revenge,” he said.