* Sandy mixing with Arctic air mass over eastern U.S.
* Similar to ``Perfect Storm'' that formed in 1991
* Large swath of U.S. East Coast potentially in harm's way
* Storm affecting presidential campaigning
(Adds new death toll, storm details)
NASSAU, Oct 26 (Reuters) - Hurricane Sandy, a late-season Atlantic storm unlike anything seen in more than two decades, slogged slowly toward the U.S. East Coast on Friday after killing at least 41 people as it cut across the Caribbean.
Forecasters said Sandy, with an expanding wind field already 550 miles (890 km) wide, had begun merging with a polar air mass over the eastern United States, potentially spawning a hybrid ``super storm'' that could wreak havoc along the East Coast.
``Its structure is evolving as we speak because it's interacting with this weather feature at higher levels of the atmosphere,'' said Todd Kimberlain, a forecaster at the U.S. National Hurricane Center in Miami.
``The models are suggesting that the storm could actually become better organized or intensify a little bit, not due to the normal processes than we would expect for a tropical cyclone but more related to this weather feature,'' Kimberlain said.
On its current projected track, government forecasters said Sandy could make landfall early next week anywhere between Virginia, Maryland or Delaware up through New York or southern New England.
In New York City, the global financial hub, officials were considering closing down mass transit before the storm hits.
Coming in the final weeks before the U.S. presidential election on Nov. 6, the storm could throw last-minute campaign travel plans into chaos.
An aide to Republican presidential challenger Mitt Romney said he had canceled a campaign event scheduled for Sunday night in Virginia Beach, Virginia. The Obama campaign said it was closely monitoring the storm.
The Democratic incumbent was traveling to New Hampshire on Saturday and on Monday was due to visit Youngstown, Ohio, and Orlando, Florida.
Much of Florida's northeast coast was under a tropical storm warning on Friday, and storm watches extended up the coast through North Carolina. Winds and rains generated by Sandy were being felt across much of Florida, with schools closed and air travel snarled in many areas.
Sandy weakened to a Category 1 storm as it tore though sparsely populated low-lying southeastern islands in the Bahamas late Thursday, knocking out power and blowing rooftops off some homes.
One storm-related death was reported in the Bahamas. Police said it was under investigation, but it occurred in Lyford Cay, a wealthy enclave of New Providence island that is home to the likes of actor Sean Connery, hedge fund billionaire Louis Bacon and Canadian fashion mogul Peter Nygard.
Sandy's driving rains and heavy winds were blamed for 40 deaths elsewhere in the Caribbean, where landslides and flash floods were triggered by the cyclone.
The Cuban government said Sandy killed 11 people when it tore across the island on Thursday. The storm took at least 26 other lives in deeply impoverished Haiti and three people were killed in neighboring Dominican Republic and Jamaica.
The Haitian dead included a family of five in Grand-Goave, west of the capital Port-au-Prince, killed in a landslide that destroyed their home, authorities said.
The Cuban fatalities were unusual for the communist ruled country that has long prided itself on protecting its people from storms by ordering mass evacuations.
The National Hurricane Center said Sandy was about 430 miles (695 km) south-southeast of Charleston, South Carolina, on Friday afternoon packing top sustained winds of 75 miles per hour (120 km per hour).
'A CATASTROPHIC STORM'
Sandy was forecast to remain a Category 1 hurricane as it moves over the Bahamas, sending swirling rains and winds across areas including Florida.
The Bahamas Electricity Corporation, which supplies power to most of the Bahamas, said Sandy had caused power outages on several of the islands.
Sandy was expected to move past the Bahamas by Friday evening and churn north toward the U.S. coast. Its speed was 7 mph (11 kph), however, making its final trek across the central and northwest corner of the Bahamas islands very slow going.
Forecasters said Sandy was expected to come ashore in the United States late Monday or early Tuesday.
``A large number of folks over a very large area of many, many states are going to experience a significant wind event of strong tropical-force winds to perhaps near hurricane-force winds covering a large area,'' said James Franklin, branch chief of the National Hurricane Center.
Storm surge and coastal or inland flooding were also big causes for concern, forecasters said.
``It's going to be a long-lasting event, two to three days of impacts for a lot of people,'' Franklin said.
Sandy, dubbed ``Frankenstorm'' by one government forecaster, was unique because of its integration with the polar trough over the United States.
``We went through this same sort of thing back about 20 years ago around Halloween in 1991 with the 'Perfect Storm,''' Kimberlain said.
That storm, featured in a book and movie of that name, combined several different storm systems to ravage the East Coast.
Some forecasters are warning that Sandy could be more destructive than last year's Hurricane Irene, which caused billions of dollars in damage as it battered the U.S. Northeast.
``This will then be a catastrophic storm for the Middle Atlantic and Northeast. It will not be a purely tropical system, with a core of powerful winds near the center, but rather more like a Nor'easter, with strong winds over a larger area,'' said a forecast report on Friday from AccuWeather.com.
``Damaging winds will affect areas from Virginia up into New York and New England, leading to widespread power outages and property damage,'' it said.
At $4.3 billion in losses, last year's Irene ranks as one of the 10 costliest hurricanes, according to the Insurance Information Institute, an industry group..
Along North Carolina's fragile Outer Banks barrier islands that jut out into the Atlantic, vacationers in large camper trailers and motor homes streamed out on Friday and the National Park Service prepared to close beaches to off-road vehicles in preparation for gale-force winds and storm surges.
Winds over 50 mph (80 kph) were expected by the weekend, causing strong beach erosion and ocean overwash on Hatteras Island's only highway as Sandy skirts by offshore, Dare County Emergency Management officials said.
Visitors centers and beaches in the Cape Hatteras National Seashore, which attracts thousands of vacationers in the summer but is less crowded in the fall, were to close late on Saturday and remain shut until the storm passes, officials said.
(Additional reporting by Jeff Franks and Nelson Acosta in Havana, Kevin Gray in Miami, Susana Ferreira in Port-au-Prince and Gene Cherry on Hatteras Island, North Carolina; Writing by Tom Brown; Editing by Vicki Allen and Doina Chiacu)