Barack Obama has warned Americans to take Hurricane Sandy seriously as authorities started shutting down the eastern seaboard ahead of its arrival.
Several states have declared emergencies, with tens of millions of people affected as schools are closed and transport services suspended.
Experts fear Sandy may become a super-storm when it makes landfall later.
Some election rallies have been called off, with Mr Obama warning affected citizens to take precautions.
International travel has been badly affected. Air France, British Airways and Virgin Atlantic grounded Monday’s transatlantic flights to and from East Coast cities, including New York, Baltimore, Newark, Washington, Boston and Philadelphia.
Full moon
At 02:00 EDT (06:00 GMT), the storm was turning north, its eye swirling about 425 miles (760km) south-east of New York City, according to the National Hurricane Center.
With winds of 75mph, Hurricane Sandy, dubbed “Frankenstorm”, is expected to bring a “life-threatening” surge flood to the mid-Atlantic coast, including Long Island Sound and New York Harbour.
The winds are expected to strengthen when Sandy makes landfall anywhere between Virginia and southern New England on Monday.
The prospect of merging with a wintry storm coming from the west during a full moon has many fearing dangerous high tides.
Sandy is some 520 miles (835km) across. It is also very slow, moving north-east at just 15mph, and could linger over as many as 12 states for 24-36 hours, bringing up to 25cm of rain, 60cm of snow, extreme storm surges and power cuts.
States of emergency have been declared in Maryland, New York, Pennsylvania, Virginia, Washington DC and parts of North Carolina.
The two presidential election contenders have modified their campaign engagements, with Mitt Romney pulling out of an event in Virginia and Mr Obama cancelling rallies in Virginia and Colorado.
The president has pulled out of a Monday event in Ohio – considered a key swing state – in order to return to Washington to monitor the storm – although he is still set to attend a rally with former President Bill Clinton in Florida earlier on Monday.
Visiting the Federal Emergency Management Agency (Fema) in Washington on Sunday, Mr Obama vowed his government would “respond big and respond fast” after Sandy had passed.
Liberty delayed
Amtrak has started suspending passenger train services across the north-eastern US and air travel has been badly hit, with some 6,800 flights cancelled.
New York City’s subway, bus and train services were suspended from 19:00 (23:00 GMT) on Sunday, and schools will be shut on Monday.
With predicted storm surges of up to 11ft, New York Mayor Michael Bloomberg ordered 375,000 people in the city’s vulnerable low-lying areas to leave their homes.
Evacuation shelters have been set up at 76 public schools.
“If you don’t evacuate you’re not just putting your own life in danger, you are also endangering lives of our first responders who would have to rescue you,” he said.
