* Flood potential called ``serious as a heart attack''
* ``Extreme rainfall'' cited as biggest threat
* Storm may halt mass transit in global financial hub
(Updates throughout with new storm information, details)
HATTERAS ISLAND, N.C., Oct 27 (Reuters) - Hurricane Sandy closed in on the United States on Saturday as it threatened to hit the eastern third of the country with torrential rains, high winds, major flooding and power outages a week before U.S. presidential and congressional elections.
A massive but slow-moving storm, with tropical storm-force winds extending across 650 miles (1,050 km), forecasters warn Sandy's flood impact could span multiple tides with a storm surge of 4 to 8 feet (1.2-2.4 meters) in Long Island Sound, the southern portion of Lower New York Bay and Delaware Bay.
Rain accumulations of up to 12 inches (30 cm) were likely in some areas.
As it merges with an Arctic jet stream, forecasters said Sandy had all the ingredients to morph into a so-called ``super storm,'' unlike anything seen over the eastern United States in decades.
Coastal flooding posed a major threat, particularly in low-lying areas like New York City, the global financial nerve center, and Alexandria, Virginia, across the Potomac River from Washington D.C.
That threat was described in a blog posted on Weather Underground (www.wunderground.com) by veteran weather forecaster Bryan Norcross as ``serious as a heart attack for anybody near the rising water.''
Governors in states along the U.S. East Coast declared emergencies, with officials urging residents to stock up on food, water and batteries.
Coming in the hectic run-up to the U.S. presidential election on Nov. 6, the storm presented a challenge to the campaigns of President Barack Obama and his Republican challenger, Mitt Romney.
As Sandy approached, Romney was rescheduling all of his campaign events planned for Virginia on Sunday and flying to Ohio instead. And Obama's campaign announced that Vice President Joe Biden had canceled a Saturday trip to Virginia Beach.
Ahead of the election, millions of Americans are taking advantage of early voting arrangements to cast their ballots. State officials said they had put in place contingency plans in case Sandy caused extended power outages or other problems that could disrupt voting.
The White House said the president convened a call with Homeland Security Secretary Janet Napolitano, Federal Emergency Management Agency Administrator Craig Fugate and other officials to receive a Saturday update on ongoing government actions to prepare for the storm.
In New York, authorities were considering closing down the city's buses, subways, commuter railroads, bridges and tunnels in preparations for the storm's onslaught.
A decision on the transportation system was likely to come on Sunday, said state operations director Howard Glaser.
A potential shutdown could begin at 7 p.m. on Sunday, when the last commuter trains would depart, with the entire system to be closed down by 3 a.m. Monday, officials said.
Sandy was about 335 miles (540 km) southeast of Charleston, South Carolina, and packing top sustained winds of 75 miles (120 km) per hour on Saturday afternoon, the National Hurricane Center said. The cyclone had briefly dropped just below hurricane strength early Saturday.
Little overall change in strength was expected ahead of its anticipated U.S. landfall early next week, the Miami-based Hurricane Center said.
The storm picked up a little forward speed but was still moving slowly over the Atlantic at 11 mph (18 kph). A jog east late Saturday morning briefly took Sandy further out to sea.
The storm's windfield has continued to expand, with hurricane force winds now extending 105 miles (165 km) from its center, government forecasters said.
Gale force winds were expected to reach portions of the mid-Atlantic coast by late Sunday and would begin hitting New York's Long Island and southern New England by Monday morning, the National Hurricane Center said.
``It's hard to imagine how millions of people are not going to be without power for an extended period of time,'' Norcross said.
BATTERED CUBA, BAHAMAS
``Regardless of the exact landfall spot this system has ... much of New England and the mid-Atlantic states are going to be impacted, perhaps very severely, by this storm,'' National Hurricane center meteorologist Chris Landsea told Reuters.
``It's certainly going to be a very significant storm when it gets up to the mid-Atlantic states,'' he added.
Sandy battered the Bahamas southeast of Florida on Friday after causing widespread destruction in eastern Cuba a day earlier. Th e storm's powerful winds and rains were blamed for at least 41 deaths in several Caribbean countries, including 11 in Cuba. Most were killed by falling trees and building collapses.
On its current projected track, Sandy could make U.S. landfall on Monday night or Tuesday anywhere between Maryland and southern New England, forecasters said.
``Perhaps the biggest concern, at the very end, may be the extreme rainfall that's going to occur after landfall,'' Landsea said. In addition to coastal and inland flooding, along with widespread power outages, Sandy was expected to dump heavy wet snow in southwest Pennsylvania and as far inland as Ohio.
High winds also threaten to disrupt air travel along the U.S. East Coast.
Tropical storm warnings and watches along Florida's east coast were lifted on Saturday as the storm moved north.
Tropical storm-force winds were being felt near the North Carolina coast and tropical storm warnings for all of the coastal portion of the state, along with about half of South Carolina, were in effect.
Along North Carolina's Outer Banks barrier islands, which jut out into the Atlantic, residents and officials said they were taking a wait-and-see approach to the storm.
As the winds and rains increased Saturday, ferry service between Ocracoke and Hatteras Islands on the Outer banks was suspended due to water on Ocracoke's only highway.
``Right now it's blowing pretty hard,'' said Ray Waller, manager of the Ocracoke office of North Carolina Ferry Dvisioin.
Outer Banks residents, with memories of damaging flooding from last year's Hurricane Irene, moved vehicles to higher ground and secured outside objects ahead of winds of more than 60 mph (96 kph) beginning Saturday night and potentially lasting into Monday.
A buoy 225 miles (362 km) south of Cape Hatteras recorded 26-foot (8-metre) waves amid blistering wind gusts early on Saturday, authorities said.
Many forecasters are warning that Sandy could be more destructive than Irene, which caused billions of dollars in damage across the U.S. Northeast.
(Additional reporting by Tom Brown in Miami, Ellen Wulfhorst in New York and Sam Youngman in Washington; Writing by Tom Brown; Editing by Vicki Allen and Todd Eastham)