* Hurricane expected ashore Monday evening
* It could be biggest to hit U.S. mainland
* New York, Philadelphia and Washington in its path
* New York Stock Exchange closed Monday and Tuesday
* Early warnings of $20 billion in economic losses (Adds comments throughout, updates storm details)
By Greg Roumeliotis and Tom Hals
NEW YORK/REHOBOTH BEACH, Del., Oct 29 (Reuters) - Hurricane Sandy, one of the biggest storms ever to hit the United States, lashed the densely populated East Coast on Monday, shutting down transportation, forcing evacuations in flood-prone areas and interrupting the presidential campaign.
Fierce winds and flooding racked hundreds of miles of Atlantic coastline and heavy snows were forecast farther inland at higher elevations as the center of the storm moves ashore along the coast of southern New Jersey or Delaware on Monday evening.
U.S. stock markets were closed for the first time since the attacks of Sept. 11, 2001, and will remain shut on Tuesday. The federal government in Washington was closed and schools were shut up and down the East Coast.
The storm's target area included big population centers such as New York City, Washington, Baltimore and Philadelphia.
More than a million customers already were without power by early evening and millions more could lose electricity. One disaster forecasting company predicted economic losses could ultimately reach $20 billion, only half of it insured.
In Washington, President Barack Obama appealed to the tens of millions of people in the hurricane's path to follow directions given to them by authorities.
"If the public's not following instructions, that makes it more dangerous for people, and it means that we could have fatalities that could have been avoided," Obama said at the White House, adding that people should expect long power outages and idled transportation systems.
New York City evacuated neighbors of a 90-story super luxury apartment building under construction after its crane partially collapsed in high winds, prompting fears the entire rig could crash to the ground.
PRESIDENTIAL CAMPAIGN INTERRUPTED
The storm interrupted the U.S. presidential campaign with eight days to go before the election, as President Barack Obama and Republican challenger Mitt Romney canceled events. Both men acted cautiously to avoid coming across as overtly political while millions of people are imperiled by the storm ahead of the Nov. 6 election.
New York and other cities closed their transit systems and schools, ordering mass evacuations from low-lying areas ahead of a storm surge that could reach as high as 11 feet (3.4 meters).
Sandy was moving quickly toward New Jersey and Delaware. At 5 p.m. (2100 GMT), the hurricane was 30 miles (45 km) east-southeast of Cape May, New Jersey, and about 40 miles (65 km) south of Atlantic City, New Jersey, the National Hurricane Center said.
Sandy picked up speed as it raced northwest toward the U.S. coast at 28 miles per hour (45 km per hour), with top sustained winds at 90 mph (150 kph), it said. Forecasters said Sandy was a rare, hybrid "super storm" created by an Arctic jet stream wrapping itself around a tropical storm.
The combination of those two storms would have been bad enough, but meteorologists said there was a third storm at play - a system coming down from Canada that would effectively trap the hurricane-nor'easter combo and hold it in place.
Moreover, the storm was coming ashore at high tide, which was pulled even higher by a full moon.
While Sandy does not have the intensity of Hurricane Katrina, which devastated New Orleans in 2005, it has been gathering strength. It killed 66 people in the Caribbean last week before pounding U.S. coastal areas as it moved north.
An AccuWeather meteorologist said Sandy "is unfolding as the Northeast's Katrina."
Forecasters said Sandy could be the largest storm to hit the mainland in U.S. history.
Off North Carolina, the U.S. Coast Guard rescued 14 of the 16 crew members who abandoned the replica tall ship HMS Bounty, using helicopters to lift them from life rafts. The Coast Guard continued to search for two missing crew members, including the captain.
Joe Connelly, 52, a trucker from the Bronx, was leaving the City Island Marina after checking on his two motor boats. He said he watched the water from the first storm-driven high tide swamp a nearby dock.
"We were concerned that the whole dock was going to float away and out to sea," he said. "It had about four feet to go before that happened."
All U.S. stock markets were closed on Monday and will remain shut on Tuesday, with a plan to re-open on Wednesday that depends on conditions after the storm passes. (Additional reporting by Greg Roumeliotis, Edith Honan, Janet McGurty, Scott DiSavino and Martinne Geller in New York, Barbara Goldberg in New Jersey, Mary Ellen Clark and Lynnley Browning in Connecticut, Daniel Lovering in Boston, Ian Simpson in West Virginia, Susan Heavey in Washington, Jane Sutton in Miami; Writing by Paul Thomasch and Ben Berkowitz; Editing by Will Dunham)