* Sandy expected to come ashore Monday night
* Sandy's large wind field a record maker
* Storm may halt, flood New York City subway system
(Adds new location of storm, detail)
HATTERAS ISLAND, N.C., Oct 28 (Reuters) - Weather forecasters warned on Sunday that Hurricane Sandy will affect a large area of the U.S. East Coast but said it was too early to pinpoint where the storm, which has the potential to be the biggest to hit the mainland, would make landfall.
Government officials in several states in Sandy's path faced tough decisions on emergency plans, including mandatory evacuations in vulnerable coastal areas, and residents scrambled to buy supplies before the storm arrives on Monday night.
On its current projected track, Sandy is most likely to make U.S. landfall between Delaware and the New York/New Jersey area, forecasters said. However, the Miami-based National Hurricane Center said it could not yet predict the precise point.
``It is still too soon to focus on the exact track ... both because of forecast uncertainty and because the impacts are going to cover such a large area away from the center,'' the NHC said in an advisory.
While Sandy's winds were not overwhelming for a hurricane, its width was what made it exceptional. Hurricane force winds extended 105 miles (165 km) from its center while its lesser tropical storm-force winds reached across 700 miles (1,125 km).
Sandy could have a brutal impact on major cities in the target zone. In New York, city officials discussed whether to shut the subway system on Sunday in advance of the storm, which could bring the country's financial nerve center to a standstill.
The storm could cause the worst flooding Connecticut has seen in more than 70 years, said the state's governor, Dannel P. Malloy.
Sandy was located about 260 miles (420 km) south-southeast of Cape Hatteras, North Carolina, with top sustained winds of 75 miles (120 km) per hour early Sunday, the NHC said.
The storm was moving over the Atlantic parallel to the U.S. coast at 13 mph (20 km/h), but was forecast to make a tight westerly turn toward the U.S. coast on Sunday night.
Tropical storm conditions were spreading across the coast of North Carolina on Sunday morning and gale force winds are forecast to begin affecting the New York area and southern New England by Monday morning, the NHC added.
Sandy could be the largest storm to hit the United States, according to the National Oceanic and Atmospheric Administration's website.
``The size of this alone, affecting a heavily populated area, is going to be history making,'' said Jeff Masters, a hurricane specialist who writes a blog posted on the Weather Underground (www.wunderground.com).
Sandy could hit Boston, New York, Baltimore, Washington, D.C. and Philadelphia, one of the most densely populated regions of the country and home to tens of millions of people.
Forecasters said Sandy was a rare, hybrid ``super storm'' created by an Arctic jet stream wrapping itself around a tropical storm, possibly causing up to 12 inches (30 cm) of rain in some areas, as well as heavy snowfall inland.
Sandy killed at least 66 people as it made its way through the Caribbean islands, including 51 in Haiti, mostly from flash flooding and mudslides, according to authorities.
The approaching storm forced a change of plans for both presidential candidates ahead of the Nov. 6 election. The White House said President Obama canceled a campaign appearance in Virginia on Monday and another stop in Colorado on Tuesday, and will instead monitor the storm from Washington.
Republican challenger Mitt Romney rescheduled campaign events planned for Virginia on Sunday and was flying to Ohio instead.
All along the U.S. coast worried residents packed stores, buying generators, candles, food and other supplies in anticipation of power outages. Some local governments announced schools would be closed on Monday and Tuesday.
``They're freaking out,'' said Joe Dautel, a clerk at a hardware store in Glenside, Pennsylvania. ``I'm selling people four, five, six packs of batteries - when I had them.''
(Additional reporting by Dave Warner in Philadelphia, Ellen Wulfhorst in New York, Mary Ellen Clark and Ebong Udoma in Connecticut and Sam Youngman in Washington; Writing by David Adams; Editing by Bill Trott/David Stamp)