Sandy strengthened before dawn and stayed on a predicted path toward Washington, Baltimore, Philadelphia and New York — putting it on a collision course with two other weather systems that would create a superstorm with the potential for havoc over 800 miles from the East Coast to the Great Lakes. About 2 to 3 feet of snow were even forecast for mountainous parts of West Virginia.

The tempest could endanger up to 50 million people for days.

Many workers planned to stay home Monday as subways, buses and trains shut down across the region under the threat of flooding that could inundate tracks and tunnels. Airports also closed, and authorities warned that the time for evacuation was running out or already past. Utilities brought in extra crews, anticipating widespread power failures.

The center of the storm was positioned to come ashore Monday night in New Jersey, meaning the worst of the surge could be in the northern part of that state and in New York City and on Long Island. Higher tides brought by a full moon compounded the threat to the metropolitan area of about 20 million people.

"This is the worst-case scenario," said Louis Uccellini, environmental prediction chief for the National Oceanic and Atmospheric Administration.

As rain from the leading edges began to fall over the Northeast on Sunday, hundreds of thousands of people from Maryland to Connecticut were ordered to leave low-lying coastal areas, including 375,000 in lower Manhattan and other parts of New York City, 50,000 in Delaware and 30,000 in Atlantic City, N.J., where the city's 12 casinos shut down for only the fourth time ever.

"I think this one's going to do us in," said Mark Palazzolo, who boarded up his bait-and-tackle shop in Point Pleasant Beach, N.J., with the same wood he used in past storms, crossing out the names of Hurricanes Isaac and Irene and spray-painting "Sandy" next to them. "I got a call from a friend of mine from Florida last night who said, 'Mark, get out! If it's not the storm, it'll be the aftermath. People are going to be fighting in the streets over gasoline and food.'"

President Barack Obama declared emergencies in Massachusetts, Connecticut, Rhode Island, New York, New Jersey and Pennsylvania, authorizing federal relief work to begin well ahead of time. He promised the government would "respond big and respond fast" after the storm hits.

"My message to the governors as well as to the mayors is anything they need, we will be there, and we will cut through red tape," Obama said. "We are not going to get bogged down with a lot of rules."

Authorities warned that New York could get hit with a surge of seawater that could swamp parts of lower Manhattan, flood subway tunnels and cripple the network of electrical and communications lines that are vital to the nation's financial center.

Major U.S. financial markets, including the New York Stock Exchange, Nasdaq and CME Group in Chicago, planned a rare shutdown Monday. The NYSE closed on Sept. 27, 1985, for Hurricane Gloria. The United Nations also shut down and canceled all meetings at its New York headquarters.

New York called off school Monday for the city's 1.1 million students and announced it would suspend all train, bus and subway service Sunday night. More than 5 million riders a day depend on the transit system.

"If you don't evacuate, you are not only endangering your life, you are also endangering the lives of the first responders who are going in to rescue you," Mayor Michael Bloomberg warned. "This is a serious and dangerous storm."

New Jersey Gov. Chris Christie was typically blunt: "Don't be stupid. Get out."

Wary of being seen as putting their political pursuits ahead of public safety, Obama and Republican nominee Mitt Romney reshuffled campaign plans as the storm approached.

In Virginia, one of the most competitive states, election officials eased absentee voting requirements for those affected by the storm. Early voting was canceled Monday in Maryland and the District of Columbia.

Sandy, a Category 1 hurricane with sustained winds of 85 mph early Monday, was blamed for 65 deaths in the Caribbean before it began traveling northward, parallel to the Eastern Seaboard. As of 5 a.m. Monday, it was centered about 385 miles south-southeast of New York City, moving to the north at 15 mph, with hurricane-force winds extending an unusual 175 miles from its center.

Gale-force winds blew overnight over coastal North Carolina, southeastern Virginia, the Delmarva Peninsula and coastal New Jersey.

Sandy was expected to hook inland Monday, colliding with a wintry storm moving in from the west and cold air streaming down from the Arctic, and then cut across into Pennsylvania and travel up through New York state.

Forecasters said the combination could bring close to a foot of rain in places, a potentially lethal storm surge of 4 to 11 feet across much of the region, and punishing winds that could cause widespread power outages that last for days. The storm could also dump up to 2 feet of snow in Kentucky, North Carolina and West Virginia.

Airlines canceled nearly 7,500 flights and Amtrak began suspending train service across the Northeast. New York, Philadelphia, Washington and Baltimore moved to shut down their subways, buses and trains. Those cities shut down their schools, as did Boston. Non-essential government offices closed in the nation's capital.

Pennsylvania's largest utilities brought in hundreds of line and tree-trimming crews in anticipation of several days of power failures or intentional shutdowns in areas with heavy flooding.

In New Jersey, where utilities were widely criticized last year for slow responses after the remnants of storms Irene and Lee, authorities promised a better performance. Bob Hanna, president of the state's Board of Public Utilities, noted that this storm has the potential to threaten even large transmission lines.

Nearly 100 miles off the coast of Cape Hatteras, N.C., a replica of the tall ship made famous in the film "Mutiny on the Bounty" was taking on water and without propulsion with 17 people aboard, the Coast Guard said. Officials were monitoring the situation early Monday.

On Sunday evening in Rehoboth, Del., only a few cars rolled along Route 1, an artery that is often bumper-to-bumper in summer.

"We were told to get the heck out. I was going to stay, but it's better to be safe than sorry," said Hugh Phillips, who was one of the first in line when a Red Cross shelter opened Sunday afternoon in neighboring Lewes.

Despite the dire warnings, some refused to budge.

Jonas Clark of Manchester Township, N.J. — right in Sandy's projected path — stood outside a convenience store, calmly sipping a coffee and wondering why people were working themselves "into a tizzy."

"I've seen a lot of major storms in my time, and there's nothing you can do but take reasonable precautions and ride out things the best you can," said Clark, 73.

The storm threatened to drench areas still recovering from last year's deluges.

In Pompton Lakes, N.J., where record flooding inundated homes a year ago, some residents were already putting belongings out near the curb in advance of the storm.

"They're figuring, divide and conquer," said resident Kevin Gogots. "They'll take the stuff they want to save and put the rest out. Of course, if the street floods again, we'll just have things floating around."