Romney has tried to expand the battlefield over the past week to states that had been considered beyond his reach.

US Republican presidential nominee and former Massachusetts Governor Mitt Romney arrives at a campaign rally in Newington, New Hampshire, Nov 3, 2012. [Photo/Agencies]

"We win Pennsylvania, we save America in three days," Romney's vice-presidential running mate, Paul Ryan, said at an airport rally in the state capital, Harrisburg.

Ryan is due to visit Minnesota on Sunday, another state that has been considered solidly Democratic. Romney himself is due to speak in Pennsylvania on Sunday. Obama officials say the Romney campaign is visiting those states out of desperation because he has been unable to establish a clear lead in other battleground states.

Nevertheless, the Obama campaign is dispatching Vice-President Joe Biden's wife, Jill Biden, to Pennsylvania and former US president Bill Clinton to Minnesota.

Obama started the day at the federal government's disaster-relief headquarters in Washington, where he received an update on the efforts to help Northeastern coastal states recover from devastating storm Sandy.

The storm has afforded the Democrat an opportunity to rise above the fray of campaigning. But it has also raised the stakes for him to show his administration can respond quickly and effectively in a crisis, as residents of New York and New Jersey vent frustration at power outages and gasoline shortages.

"He's focused on it every moment he's not speaking on the stage," Obama campaign spokeswoman Jen Psaki told reporters aboard Air Force One.