* Flood potential called ``serious as a heart attack''
* ``Extreme rainfall'' cited as biggest threat
* Sandy killed at least 41 across the Caribbean
(Recasts throughout, changes dateline and byline, previous MIAMI)
HATTERAS ISLAND, N.C., Oct 27 (Reuters) - Hurricane Sandy closed in on the United States on Saturday, where it threatens 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.
The late-season storm has been dubbed ``Frankenstorm'' by some weather watchers because it will combine elements of a tropical cyclone and a winter storm and is forecast to reach the U.S. coast close to Halloween.
As it merges with an Arctic air mass high over the eastern United States, forecast models show it will have all the ingredients to morph into a ``super storm.''
Coastal flooding posed a major threat, particularly to low-lying areas like New York City, the global financial nerve center.
That threat was described in blog posted on Weather Underground (www.weatherunderground.com) on Saturday 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 on Friday, 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.
Romney canceled a rally scheduled for Sunday evening in Virginia Beach, Virginia, while Obama's re-election campaign announced that Vice President Joe Biden had also canceled a Saturday trip to that city.
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.
In New York City, officials were considering shutting down the country's largest mass transit system because of concerns the storm could cause flooding or high winds that would make subway and bus travel perilous.
Sandy was about 355 miles (570 km) southeast of Charleston, South Carolina, and packing top sustained winds of 75 miles (120 km) late Saturday morning, the U.S. National Hurricane Center said. It had briefly dropped just below hurricane strength.
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 overnight but was still moving slowly over the Atlantic at 9 mph (15 kph). A jog east late Saturday morning briefly took Sandy further out to sea.
The massive storm has continued to expand, with tropical force winds now extending 450 miles (725 km) from its center, government forecasters 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.
The 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 took a wait and see approach to the storm as rain fell and the winds and surf picked up early Saturday.
``We're watching it and waiting and seeing,'' said North Carolina Emergency Management spokeswoman Julia Jarema.
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.
Beach erosion and ocean overwash of the only highway on Hatteras Island were expected, shutting off several thousand year-round residents from the mainland.
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 last year's Hurricane Irene, which caused billions of dollars in damage across the U.S. Northeast.
(Additional reporting by Tom Brown in Miami, Desmond Boylan in Cuba, Neil Hartnell in Nassau, Patricia Zengerle in Washington, Barbara Goldberg in New York and Gene Cherry on Hatteras Island, North Carolina; Editing by David Adams, Andrew Osborn and Vicki Allen)