The approach of Hurricane Sandy reminded them just how out of their control democracy can be.

“Obviously, we believe the more people participate in the election, the better,” said David Axelrod, the president’s senior strategist, “and the storm can be disruptive. But to the 50 million people in its path, there are more immediate and potentially grave concerns that transcend politics. We’ll have to wait and see its impact.”

The storm forced both candidates to scrap campaign stops and, with eight days until Election Day, will require Mr. Obama to balance the roles of president in an emergency and candidate. That could benefit or hurt him, depending on how voters view his performance, and distract from efforts by both camps to advance a closing argument.

Early voting, which Mr. Obama has counted on to bolster his chances of a second term, will most likely grind to a halt in some places along the Eastern Seaboard, while power failures could last much of the week and conceivably until Election Day in some places. It went unnoticed by no one that Virginia, among the most tightly contested states, may be among the most affected.

Meteorology is only one wild card facing the campaigns in the final week. On Election Day, the winner may not be known right away; results in one or more states may be close enough to merit recounts. In Ohio, which could decide the election, so many provisional ballots may be cast — and by law are not counted right away — that it may be mid-November before a winner is declared.

“The Boy Scout motto comes in handy — be prepared,” said Bradley Blakeman, a Republican strategist and veteran of George W. Bush’s recount fight in Florida. “I know that lists of local lawyers and national legal talent are amassed and will be deployed if need be. After the recount in 2000 and the nail-biter in 2004, the G.O.P. is ready with multiple scenarios already modeled.”