Hurricane Sandy is swirling towards the US, forcing presidential candidates to adjust schedules and cancel events.
President Barack Obama has held a conference call with emergency chiefs to discuss preparations for the storm, which could hit as early as Monday.
Its sustained winds of 75mph (120km/h) are set to intensify as it merges with a wintry storm from the western US.
New York City is to suspend its subway, bus and train services from Sunday evening, the governor has announced.
A number of states key to the election could be hit by a storm that may affect up to 60 million Americans.
At 08:00 EDT (12:00 GMT), the eye of the storm was about 260 miles (320km) South of Cape Hatteras in North Carolina, according to the National Hurricane Center. It was expected to bring a "life-threatening" surge flooding to the Mid-Atlantic coast.
Craig Fugate, director of the Federal Emergency Management Agency, said: "This is not a coastal threat alone. This is a very large area."
Sandy has already killed 60 people in the Caribbean during the past week.
Republican candidate Mitt Romney cancelled an event scheduled for Sunday in Virginia, a key election state, because of the weather, and was instead heading to Ohio.
President Obama will head to Florida on Sunday rather than Monday, and has cancelled a campaign stop with former President Bill Clinton in Virginia on Monday and a rally in Colorado on Tuesday to monitor the storm from the White House, said White House spokesman Jay Carney.
Vice-President Joe Biden also cancelled a rally in coastal Virginia to allow for disaster preparations.
Early balloting in Maryland saw lines of voters stretching for a number of blocks at some polling stations on Saturday.
But despite concerns about Sandy's impact, with some polls suggesting the contest is a virtual dead heat, both Mr Romney and Mr Obama pressed ahead with campaigning in key swing states on Saturday.
Nine states are thought to be too close to call.
In New Hampshire, Mr Obama urged his supporters to encourage people to vote early and allow him to finish the job he started.
"We've still got a lot of work to do, but New Hampshire and the country has come too far to go back to the policies that got us into this mess," he said.
"All he's offering is a big rerun of the same policies," Mr Obama said of his opponent.
In Florida, Mr Romney said he stood for "big ideas" that would get America going again, compared to what he called Mr Obama's "shrinking agenda".
"The president doesn't have a plan, he's out of ideas, he's out of excuses and this November, Florida is going to make sure we put him out of office," Mr Romney said to cheers from the conservative crowd in Pensacola.
How Mr Obama handles the weather emergency and how far Mr Romney tries to make political capital out of it could enhance or harm their chances, says the BBC's Bridget Kendall, on the election trail.
'Prepare for the worst'
While the East Coast is used to extreme weather, Sandy is concerning meteorologists who fear it could mutate into a "Frankenstorm" as it merges with a winter storm in the run-up to Halloween.
It is only moving north-east at 13mph, meaning it could hover for 36 hours over as many as 12 states, bringing up to 10in (25cm) of rain, 2ft of snow, extreme storm surges and power cuts.
States of emergency have been declared in Maryland, New York, Pennsylvania, Virginia, Washington DC and a coastal county in North Carolina.
Tropical storm warnings are in effect in both South and North Carolina, as well as Pamlico and Albemarle Sounds.
The NHC said further strengthening was possible on Sunday, before Sandy touched down anywhere between Virginia and southern New England late on Monday.
In New Jersey, Governor Chris Christie pleaded with residents not to be complacent.
"I know everyone's saying this isn't going to happen… that the weathermen always get it wrong," he said.
He urged people to stock up on essentials in case they were trapped at home for a few days.
"We have to be prepared for the worst here. I can be as cynical as any of you but when the storm comes, if it's as bad as they're predicting it will be, you're gonna wish you weren't as cynical as you might otherwise have been."
Delaware has ordered a mandatory evacuation of 50,000 people from coastal areas.
New York has not yet ordered evacuations.
Mayor Michael Bloomberg said: "This is a dangerous storm. But I think we're going to be OK."
Earlier in the week, Sandy caused havoc as it ploughed across the Caribbean, killing at least 44 people in Haiti, 11 in Cuba and four more in the Dominican Republic, Jamaica and the Bahamas.