Last Updated 11:15 a.m. ET
Hurricane Sandy, the mammoth storm that has already claimed more than five dozen lives in the Caribbean, could potentially affect some 60 million people in the Eastern United States, where it will meet with two other powerful winter storm systems, to create an even more threatening hybrid.
Sandy is expected to afflict a third of the country with sheets of rain, high winds and heavy snow, say officials who warned millions in coastal areas to get out of the way.
"We're looking at impact of greater than 50 to 60 million people," said Louis Uccellini, head of environmental prediction for the National Oceanic and Atmospheric Administration.
Experts said it didn't matter how strong Sandy is when it hits land: The rare hybrid storm that follows will cause havoc over 800 miles, from the East Coast to the Great Lakes. States of Emergency have been declared from North Carolina up to New England.
It's expected to make landfall early Tuesday morning around the coast of Delaware, with storm surges from 4 to 8 feet in some areas, meaning evacuations are likely.
The National Hurricane Center says "life-threatening storm surge flooding" is expected along the Mid-Atlantic coast, including Long Island Sound and New York Harbor.
Sandy is currently a Category 1 hurricane. At 11:00 a.m. Sunday morning Sandy was centered about about 250 southeast of Cape Hatteras, N.C., and about 575 miles south of New York City, moving northeast at 14 mph. Maximum sustained winds were 75 mph.
Tropical storm warnings remains in effect in North Carolina from Cape Fear to Duck North, Palmlico and Albermarle Sounds, and in Bermuda.
Officials say the storm-related death toll in the Caribbean has risen to 65 as of early Sunday. Haitian authorities have reported 51 of those deaths. Continued rains have added to the initial jolt of the hurricane. Deaths also have been reported in Cuba, Jamaica, Puerto Rico and the Bahamas.
Governors from North Carolina, where steady rains were whipped by gusting winds Saturday night, to Connecticut declared states of emergency. Delaware ordered mandatory evacuations for coastal communities by 8 p.m. Sunday.
New Jersey Gov. Chris Christie, who was criticized for not interrupting a vacation in Florida while a snowstorm pummeled the state in 2010, broke off campaigning for Republican presidential nominee Mitt Romney in North Carolina on Friday to return home.
"I can be as cynical as anyone," said Christie, who declared a state of emergency Saturday. "But when the storm comes, if it's as bad as they're predicting, you're going to wish you weren't as cynical as you otherwise might have been."
Eighty-five-year-old former sailor Ray Leonard agreed. And he knows to heed warnings.
Leonard and two crewmates in his 32-foot sailboat, Satori, rode out 1991's infamous "perfect storm," made famous by the Sebastian Junger best-selling book of the same name, before being plucked from the Atlantic off Martha's Vineyard, Mass., by a Coast Guard helicopter.
"Don't be rash," Leonard said Saturday from his home in Fort Myers, Fla. "Because if this does hit, you're going to lose all those little things you've spent the last 20 years feeling good about."
The storm was expected to continue moving parallel to the Southeast coast most of the day and approach the coast of the mid-Atlantic states by Monday night, before reaching southern New England later in the week.
It was so big, however, and the convergence of the three storms so rare, that "we just can't pinpoint who is going to get the worst of it," said Rick Knabb, director of the National Hurricane Center in Miami.
Officials are particularly worried about the possibility of subway flooding in New York City, said Uccellini, of NOAA.
New York Gov. Andrew Cuomo told the Metropolitan Transportation Authority to prepare to shut the city's subways, buses and suburban trains. The city closed the subways before Hurricane Irene last year, and a Columbia University study predicted that an Irene surge just 1 foot higher would have paralyzed lower Manhattan.
Up and down the Eastern Seaboard and far inland, officials urged residents and businesses to prepare in ways big and small.
On Saturday evening, Amtrak began canceling train service to parts of the East Coast, including between Washington, D.C., and New York. Airlines started moving planes out of airports to avoid damage and adding Sunday flights out of New York and Washington in preparation for flight cancellations on Monday.
The Virginia National Guard was authorized to call up to 500 troops to active duty for debris removal and road-clearing, while homeowners stacked sandbags at their front doors in coastal towns.
"You never want to be too naive, but ultimately, it's not in our hands anyway," said Andrew Ferencsik, 31, as he purchased plywood and 2-by-4 lumber from a Home Depot in Lewes, Del.
Utility officials warned rains could saturate the ground, causing trees to topple into power lines, and told residents to prepare for several days at home without power.
President Barack Obama was monitoring the storm and working with state and locals governments to make sure they get the resources needed to prepare, administration officials said.
In North Carolina's Outer Banks, a group of about 20 people was forced to wait out the storm on Portsmouth Island, a former fishing village that is now uninhabited and accessible only by private ferry.
"We tried to get off the island and the ferry service shut down on us," said Bill Rowley, 49, of Rocky Mount, N.C.
Rowley said he could see 15-foot seas breaking over the island's dunes, enough to bring water to the island's interior.
"We'll be inundated and it'll probably be worse tomorrow," he said.
In New Jersey, hundreds of coastal residents started moving inland. Christie's emergency declaration will force the shutdown of Atlantic City's 12 casinos for only the fourth time in the 34-year history of legalized gambling here. City officials said they would begin evacuating the gambling hub's 30,000 residents at noon Sunday, busing them to mainland shelters and schools.
The storm also forced the presidential campaign to juggle schedules. Romney scrapped plans to campaign Sunday in Virginia and switched his schedule for the day to Ohio. First lady Michelle Obama canceled an appearance in New Hampshire for Tuesday, and Mr. Obama moved a planned Monday departure for Florida to Sunday night to beat the storm. He also canceled appearances in Northern Virginia on Monday and Colorado on Tuesday.