Mass transit shut down across the densely populated Northeast, landmarks stood empty and schools and government offices were closed. The National Grid, which provides power to millions of customers, said 60 million people could be affected before it's over.

On Fire Island, off Long Island, the water rose above promenades and docks on Monday afternoon, homeowner Karen Boss said. Boss stayed on the island with her husband despite a mandatory evacuation order. She said they own several properties and a business there and had weathered previous storms.

"I'm concerned that it might come into the first floor," she said. "If that's the case, I'll just move into another house that's higher up."

Based on pressure readings, it's likely to be the strongest storm to make landfall north of Cape Hatteras, North Carolina, CNN senior meteorologist Dave Hennen said. The benchmark storm, the 1938 "Long Island Express" Hurricane, contained a low pressure reading of 946 millibars; Sandy had a minimum pressure of 943 millibars. Generally speaking, the lower the pressure, the stronger the storm.

In Sea Bright, New Jersey, Yvette Cafaro scrawled a plea on the plywood that covered her burger restaurant: "Be kind to us Sandy." The seaside area largely dodged last year's Hurricane Irene, but Cafaro was not optimistic that Sea Bright would be spared Sandy.

"Everything that we've been watching on the news looks like this one will really get us," she said. "We're definitely worried about it."

Its arrival, eight days before the U.S. presidential election, forced President Barack Obama and his Republican challenger, Mitt Romney, to alter or cancel several campaign stops. Obama flew back to Washington from Florida, telling reporters at the White House that assets were in place for an effective response to the storm.

"The most important message I have for the public right now is please listen to what your state and local officials are saying," Obama said. "When they tell you to evacuate, you need to evacuate."

And in Ohio, Romney asked supporters to drop off items and cash at his "victory centers" to be donated to victims of the storm.

"There are families in harm's way that will be hurt -- either in their possessions or perhaps in something more severe," Romney said.

By Monday afternoon, 23 states were under a warning or advisory for wind related to Sandy. Thousands of flights had been canceled, and hundreds of roads and highways were expected to flood. And according to a government model, Sandy's wind damage alone could cause more than $7 billion in economic loss.

Sandy was expected to weaken once it moves inland, but the center was expected to move slowly northward, meaning gusty winds and heavy rain would continue through Wednesday.

On the western side of the storm, the mountains of West Virginia expected up to 3 feet of snow and the mountains of southwestern Virginia to the Kentucky state line could see up to 2 feet. Twelve to 18 inches of snow were expected in the mountains near the North Carolina-Tennessee border.

"This is not a typical storm," said Pennsylvania Gov. Tom Corbett. "Essentially, this is a hurricane wrapped in a 'nor'easter.'"