The declaration will allow the state to request federal funding and other assistance for actions taken in advance of the massive storm, which is expected to make landfall in New Jersey Monday night.
The storm was still hundreds of miles away, but was already making its approach known to New Jersey on Sunday with high winds, rough surf and coastal flooding as thousands of people fled to what they hoped would be safer ground. Numerous towns across the state issued voluntary evacuation orders; some made them mandatory.
Christie, who famously urged New Jersey residents last year to "get the hell off the beach" as Hurricane Irene approached, urged residents of the state's narrow barrier islands to move to higher ground. He predicted the storm would come ashore at Atlantic City around 2 a.m. Tuesday.
More than half of New Jersey's 590 school districts had already announced decisions to close on Monday, and 247 districts have decided to remain shut Tuesday, as well.
Sandy was just one component of a massive storm predicted to come together over the eastern third of the U.S., threatening damaging wind, possible record-setting flooding and prolonged power outages.
"I think this one's going to do us in," said Marc Palazzolo, owner of a bait and tackle shop on an inlet to the ocean in Point Pleasant Beach. He used the same wood he boarded up the store with in past storms to secure it this time, crossing out the names of hurricanes Isaac and Irene and spray-painting "Sandy" next to them.
"I got a call from a friend of mine from Florida last night who said, 'Marc: Get out! If it's not the storm, it'll be the aftermath. People are going to be fighting in the streets over gasoline and food," he said.
Yet Palazzolo was floored by the response to a Facebook plea he issued Saturday night for people to come help him fill sandbags to protect his shop and nearby buildings: 30 people showed up, many of them strangers. One man drove more than two hours from Bergen County to pitch in.
Christie said a southern-state landfall will cause more flooding in the Raritan River area, while a Monmouth County landfall will result in more extensive inland flooding of the Passaic River.
In Atlantic City, an evacuation of residents from the gambling resort was under way, with people lined up at the Convention Center to board buses to shelters on the mainland. Emergency officials said Sunday afternoon's low tide looked like a typical high tide, a worrisome development in this island city.
"We have to play the hand we've been dealt and unfortunately this is a bad hand," said Mayor Lorenzo Langford, himself a former casino dealer.
Denise Faulkner was getting ready to evacuate for the second time in little over a year.
"I'm real overwhelmed," she said as she waited for a bus with her 7-month-old daughter, her 3- and 12-year-old sons and her boyfriend. "We're at it again. Last year we had to do it. This year we have to do it. And you have to be around all sorts of people. Strangers. It's a bit much.
"We're leaving just to protect the kids," she said. "It looks like it's going to be real serious this year."
John Williams, his wife Robshima and their three children also waited for a bus out of Atlantic City to a shelter. The family wants to make the shelter fun; they've packed the kids' Halloween costumes and want them to dress up. One child will be the grim reaper, one a witch and one a zombie.
"We're just trying to make a bad situation good," Robshima Williams said. "This was supposed to be Halloween weekend. We're going to make it fun no matter where we are." The family, she added, "Is going to go to everybody's bunk and trick or treat."
The city's 12 casinos closed for only the fourth time in the 34-year history of legalized gambling here. State parks also shut down.
Residents of northern New Jersey river communities braced for another round of the flooding that has become commonplace for them. Pompton Lakes has been hit by flooding several times in the last decade, most notably last year after the remnants of Hurricane Irene swept through the area and left dozens of businesses and homes severely damaged.
Some in the town were already putting belongings out near the curb, in advance of the storm.
"People are worst-case-scenario-ing it," said Kevin Gogots, who has lived in the town since the early '80s. "They're figuring, divide and conquer: They'll take the stuff they want to save and put the rest out. Of course, if the street floods again we'll just have things floating around."
Low-lying areas along the Jersey shore were already taking on water Sunday because of a convergence of high winds and high tides. In Sea Bright, the Coast Guard and a fishing vessel crew rescued a surfer who couldn't make it back to shore due to wild waves and strong currents.
Associated Press writer David Porter in Pompton Lakes contributed to this story.