WASHINGTON -- Hurricane Sandy added an unprecedented dose of uncertainty to an already-unpredictable presidential race Monday, forcing President Barack Obama and Republican Mitt Romney to scramble their campaign schedules and raising the possibility that some states might have to alter Election Day plans.

Obama canceled a campaign appearance in Florida and raced back to the White House, mindful that his performance leading the federal government response could become a major boon or liability to his re-election chances. Romney canceled campaign events for Monday night and Tuesday, and both campaigns curtailed fundraising efforts in states that Sandy was expected to hit.

The Federal Emergency Management Agency said it was preparing for the possibility that widespread power outages and damage to polling places might not be remedied in some states in time for Election Day a week from Tuesday.

"We are anticipating that based on the storm, there could be impacts that would linger into next week and have impacts on federal elections," FEMA administrator Craig Fugate said. "Our chief counsel’s been working on making sure that we have the proper guidance on how to support any actions that may be required” in areas that are declared disasters.

That includes determining whether states can be reimbursed for any work they must to do to fix or move polling places damaged in the storm. "This will led by the states," Fugate said. "We’ll be in a support role."

The storm will affect campaigning. Traveling will be more difficult, and last-minute rallies, often vital for news coverage and for motivating workers, will have to be rescheduled.

Campaign workers in swing states such as Virginia, New Hampshire and perhaps Pennsylvania are likely to find it harder to knock on doors and staff phone banks. On the airwaves, campaigns might be reluctant to continue the barrage of attack ads at a time when millions are struggling to cope with Sandy’s effects.

“The last thing someone with a flooded basement wants is to deal with a barrage of volunteers,” said Brad Dayspring, a Republican consultant who worked for George W. Bush in 2000 and 2004.

“Nobody knows what’s going to happen. Will this help President Obama? Will Romney do something that looks too political? We don’t know,” veteran Democratic strategist Tad Devine said.

Obama, who’d already canceled campaign appearances scheduled for later Monday and early Tuesday, abruptly dropped his appearance at a planned rally in Orlando as well.

"The storm overnight picked up speed and intensity,” Press Secretary Jay Carney said. “And a decision was made that in order to return to Washington to monitor and oversee the efforts to prepare for the storm and respond to it, we needed to leave earlier than planned.”

The president’s quick move back to Washington was a contrast to his decision in September to attend a Las Vegas fundraiser the day after four Americans, including the U.S. ambassador, were killed in an attack on the American consulate in Libya.

At the time, senior adviser David Axelrod explained that Obama was in frequent contact with top advisers, and said, “The president of the United States is responsible for everything that happens.” On Monday, Carney brushed aside a question about why the president needed to be in the White House this time.