* Sandy expected to come ashore Monday night or Tuesday
* Sandy's large wind field a record maker
* Storm may halt, flood New York City subway system
HATTERAS ISLAND, N.C., Oct 28 (Reuters) - Weather forecasters worked to pinpoint the likely landfall of the monstrous Hurricane Sandy as it closed in on the U.S. East Coast Sunday with the potential to be the biggest storm to hit the mainland.
Government officials faced tough decisions on emergency plans as residents scrambled to purchase supplies. Governors of several states in the hurricane's path declared emergencies and ordered mandatory evacuations of vulnerable coastal areas.
On its current projected track, Sandy is most likely to make U.S. landfall on Monday night between Delaware and the New York/New Jersey area, forecasters said.
While Sandy's winds were not overwhelming for a hurricane, its width was what made it exceptional. The storm's 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 county'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.
Government forecasters at the Miami-based National Hurricane Center said as the storm approached land it became increasingly pointless to predict the precise landfall.
``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.
Sandy was located about 275 miles (445 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 14 miles per hour (22 kph), but was forecast to make a tight westerly turn toward the U.S. coast on Sunday night.
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 impact the cities of Boston, New York, Baltimore, Washington, D.C. and Philadelphia, one of the most densely populated regions of the country 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)