In the final hours of the 2012 American presidential campaign, Republican presidential candidate Mitt Romney attended campaign stops in four states on Monday, and warned voters of another recession if Democratic President Barack Obama is re-elected.

Romney's wife, Ann, joined him to rally support before a cheering crowd of more than 8,000 people at George Mason University's Patriot Centre arena in Fairfax, Virginia.

The energised crowd roared its approval when she asked the audience, "are we going to be neighbours soon?"

Romney was rallying voters across four swing states and urging them to vote on Tuesday.

Speaking to an enthusiastic crowd in Fairfax, Romney said, "We have journeyed far and wide in this campaign together for America's future and now we are almost there."

"The door to a brighter future is there, it's open, it is waiting for us."

"I need your vote. I need your work. Walk with me, let's walk together," he implored Virginian voters.

Early on Monday morning, Romney had set off on his last and longest day of campaigning, a sprint through Florida, Virginia, Ohio and New Hampshire, ending in a late-night rally in Manchester that was originally billed as his last campaign stop.

However, Romney's team then announced a last-minute election day push for Tuesday that will take him to Cleveland, Ohio and Pittsburgh, Pennsylvania for get-out-the-vote efforts before he returns to Boston, Massachusetts to await the outcome.

Ohio is a critical battleground that Romney has visited again and again, but one where polls show a race with Obama that's stubbornly close.

Romney all but ignored Pennsylvania until the final week of the campaign, as Republicans poured (m) millions into previously empty airwaves in a bid to expand the party's reach.

Romney will be banking on Ohio to carry him over the finish line in what's been a fluid but close-fought contest.

Without Ohio, Romney has to win nearly every other battleground state to defeat Obama.

The Obama campaign was in Ohio on Monday afternoon, along with some celebrity supporters.

Joining rock legend Bruce Springsteen to open Obama's Columbus rally was rapper Jay-Z, who entertained the audience before Obama stood to speak.

With a voice clearly showing the strain of weeks of campaigning, Obama urged Ohio voters to consider the future as they pushed for the finish line.

"The future never has lobbyists, like the status quo does, but the dreams of those children will be our saving grace, " he said.

"We've come too far to let our hearts grow faint."

Obama will close his 2012 campaign with a night time rally on Monday in Iowa, where his 2008 caucus victory jump started his road to the White House.

The president is expected to reflect on the state's pivotal role in his political rise during remarks delivered at the site of his first Iowa campaign headquarters.

But it's more than just nostalgia that is bringing Obama back to Iowa one more time.

The president's team is seeking to hold off Romney by sweeping the Midwest, including Iowa's relatively small prize of six Electoral College votes.

Obama will be joined by his wife, Michelle, singer Bruce Springsteen, and a cadre of long-time advisers and friends for the final stretch of his campaign.