Hurricane Sandy is heading north from the Caribbean - where it left nearly 60 people dead - to threaten the eastern US with sheets of rain, high winds and heavy snow as millions were warned to get out of its path.
Sandy is expected to affect up to 60 million people when it meets two other powerful winter storms. Experts said it will not matter how strong the storm is when it hits land - the rare hybrid that follows will cause havoc over 800 miles from the East Coast to the Great Lakes.
"This is not a coastal threat alone," said Craig Fugate, director of the Federal Emergency Management Agency. "This is a very large area."
New Jersey governor Chris Christie declared a state of emergency as hundreds of coastal residents started moving inland and the state moved to close its casinos. New York's governor is considering shutting down the subways to avoid flooding and half a dozen states warned residents to prepare for several days of lost power.
Sandy weakened briefly to a tropical storm early on Saturday but was soon back up to Category 1 hurricane strength, packing 75mph winds about 335 miles south east of Charleston, South Carolina.
Experts said the storm was most likely to hit the southern New Jersey coastline by late Monday or early Tuesday.
Governors from North Carolina, where heavy rain is expected on Sunday, to Connecticut declared states of emergency. Delaware ordered mandatory evacuations for coastal communities.
The storm forced the US presidential campaign to juggle schedules. Mitt Romney scrapped plans to campaign in the swing state of Virginia and switched his schedule for the day to Ohio. First Lady Michelle Obama cancelled an appearance in New Hampshire for Tuesday, and President Barack Obama moved a planned Monday departure for Florida to Sunday to beat the storm.
What makes the storm so dangerous and unusual is that it is coming at the tail end of hurricane season and the beginning of winter storm season, "so it's kind of taking something from both", said Jeff Masters, director of the private service Weather Underground.
Mr Masters said the storm could be bigger than the worst East Coast storm on record - the 1938 New England hurricane known as the Long Island Express, which killed nearly 800 people. Experts said to expect high winds over 800 miles and up to 2ft of snow as far inland as West Virginia.