President Barack Obama left the campaign trail Monday to lead his nation at a moment of crisis, steering the response to Hurricane Sandy, which left the endgame of the White House race in turmoil.

Republican Mitt Romney joined the president in cancelling campaign appearances as high winds, swamping tides and lashing rain hit the northeastern United States and conjured a moment of political peril for the rivals.

Coming so close to the neck-and-neck election on November 6, the potentially historic storm threw closely planned campaign strategies into disarray, could dampen early voting, and may drown out the candidates' closing arguments.

"This is going to be a big and powerful storm," Obama warned after meeting disaster and emergency officials at the White House, and ditching events in battlegrounds Florida, Ohio and Virginia and rushing back to Washington.

Obama also struck a patriotic note, striving for national unity despite sharp political divides cleaving the United States.

"The great thing about America is, when we go through tough times like this, we all pull together," Obama said, trying to project competence and authority, as he grabbed headlines with a sober televised statement at the White House.

"The election will take care of itself next week. Right now, our number one priority is to make sure that we are saving lives."

Obama was directing the government response to the storm from the secure Situation Room below the White House, immediately setting up a contrast with his out-of-office challenger.

Romney cancelled campaign events later Monday in Wisconsin and all events on Tuesday as a mark of sensitivity towards millions in the path of the storm, but went ahead with scheduled appearances in Iowa and Ohio.

The Republican told supporters in Ohio that Americans on the East Coast were facing "very difficult times" in the storm.

"There are families in harm's way that will be hurt, either in their possessions or perhaps in something more severe," he said, and appealed for donations to the American Red Cross.

Romney was on a political tightrope, balancing a desire to use the precious last days of the campaign to maintain momentum, with a desire to avoid appearing oblivious to Americans affected by the hurricane.

He has already been accused of muscling in on tragedy for political gain -- over the attack on the US consulate in Benghazi last month -- and so can ill afford any missteps seen as motivated by a hope of an electoral dividend.

Equally, any errors by Obama in the wake of the storm could help Romney build his case that Benghazi was a symptom of a wider malaise and unraveling of leadership in the White House.

High-level campaign operatives deplore events they cannot control, hence the fabled history of the "October Surprise" -- the sudden happening, at home or abroad, with the potential to reshape the late stages of an election.

Though the disaster alert allowed Obama to leverage the advantage of incumbency and to showcase his leadership skills, it also left him carrying the can if the government's disaster response to the storm is revealed as lacking.

Top US office holders have been acutely aware of the potential of disasters to wreak a political price ever since president George W. Bush's bungled handling of Hurricane Katrina seven years ago.

The disastrous confluence of miscommunication and chaotic governing that left thousands stranded in New Orleans seemed unlikely to be repeated, given reforms in relief and emergency response by the Obama administration.

Local political leaders like New York Mayor Michael Bloomberg, Newark Mayor Cory Booker, Virginia Governor Robert McDonnell and New Jersey Governor Chris Christie also appear of a higher caliber than Louisiana's hapless leadership in 2005.

So it may be that the prime political impact of Sandy will weigh on the election endgame.

Romney risks becoming an afterthought, especially if there is widespread loss of life or damage, and could see the momentum he has built up in recent weeks squelched as Obama tries to shape the narrative of the storm.

The hurricane and likely widespread power cuts in swing states like Virginia will also disarm both campaigns, which had planned to deluge voters with non-stop television advertising in the final days.

When the storm brings days of disrupted weather inland, it could also dampen early voting turnout, even in places as sheltered as midwestern Ohio or battleground New Hampshire, on which the Obama campaign is counting.

Romney leads by a few points in some national polls of the popular vote, but Obama appears to be clinging to a narrow advantage in the state-by-state race to 270 electoral votes needed to secure the White House.

But Obama was up one point, a swing back to the president of three points from last week, in the latest GWU/Politico/Battleground poll Monday.

A CNN/ORC poll in Florida, the biggest swing state, meanwhile suggested the race has tightened, with Romney leading by only a single point.