"This is certainly the biggest thing I've covered in my career," Jim Cantore, a meteorologist at The Weather Channel who reported extensively on 2005's devastating hurricane Katrina, told viewers hours before the storm was set to make landfall.
Added Maryland Gov. Martin O'Malley at an afternoon news conference: "This is a serious, killer storm."
Sandy was forecast to pummel eight U.S. states over the next couple of days as it marches towards Canada, where it's expected to wreak the most havoc in eastern Ontario and western Quebec.
If, as expected, the hurricane collides with a cold front to the west and a high-pressure system from Greenland, the trio of clashing weather systems could create a "perfect storm" with the potential to pummel the Eastern Seaboard for days.
U.S. President Barack Obama visited the headquarters of the Federal Emergency Management Agency in the D.C., urging Americans to heed the advice of officials.
"This is a serious and big storm," he said.
"You need to take this very seriously and follow the instructions of your state and local officials, because they are going to be providing you with the best advice in terms of how to deal with this storm over the coming days."
Both Obama and Mitt Romney, his Republican rival for the White House, cancelled events Sunday as they headed into the last full week of the presidential election campaign.
That included scrubbing appearances in Virginia, a key swing state where the men are running neck and neck with just 10 days until the presidential election.
Both Republican and Democratic officials are worried that the mega-storm could dramatically affect voter turnout on Nov. 6 in the states of Ohio, North Carolina and Virginia — all of them critical battleground states.
That's a particular concern if those states experience extensive, long-lasting power outages of the type that occurred when a monumental summer storm pounded the Eastern Seaboard just five months ago.
"Obviously we want unfettered access to the polls because we believe the more people who come out, the better we're going to do," David Axelrod, Obama's chief strategist, said Sunday.
"The best thing we can do is to focus on how we can help people, and hope it all clears out by next weekend."
Virginia Gov. Bob McDonnell said his state is putting in place steps to recover as quickly as possible from the aftermath of the hurricane, intent that it doesn't keep voters from the polls. Obama and Romney are running neck and neck in Virginia.
"We have contingency plans in place," the Republican governor said on CNN. "We've got about 2,000 additional people coming into Virginia to help. All hands will be on deck."
Polling sites, including schools and community centres, will take precedence in terms of restoring power, he added. The state elections board also is planning to extend voting hours.
In the U.S. capital, the city's public transit system announced it was shutting down entirely on Monday. The federal government also told its employees to stay home.
Elsewhere on the East Coast, New York City shut down its subway system. The hurricane has the potential to create a storm surge that could overwhelm Manhattan's flood walls and swamp its subway tunnels.
"The storm is coming ... and now it is time to take action," Gov. Andrew Cuomo told a news conference. "This is nothing to play with, and this is nothing to take lightly. So take it seriously."
New York City Mayor Michael Bloomberg ordered the evacuation of parts of lower Manhattan and the Rockaways, a peninsula on Long Island. He also ordered New York harbour closed.
"If you don't evacuate, you are not only endangering your life, you are also endangering the lives of the first responders who are going in to rescue you," Bloomberg said.
Meteorologists and other officials have been warning citizens to prepare for the worst for days. They've also urged people to steer well clear of beaches and coastlines because of the potential of perilously high winds and mammoth waves.
On Sunday, a sea buoy located 250 kilometres off the North Carolina coast reported wave heights of almost 10 metres every 13 seconds. Wind gusts of 120 k/ph were forecast throughout the region.
The storm surges along the coast could swell higher than three metres in places, including the Long Island Sound area, posing a genuine threat to human life, said Rick Knabb, director of the U.S. National Hurricane Center.
"The system is so large that I would say millions of people are at least in areas that have some chance of experiencing either flash flooding or river flooding," Knabb said.
"The size of the storm is going to carve a pretty large swath of bad weather. This is not just a coastal event."
Coastal area residents in several states were ordered to evacuate on Sunday while school was cancelled for at least two days in various state counties, including Maryland, New Jersey, New York and Delaware.
"It has become clear that the state will be affected by high winds, heavy rainfall, and flooding, especially along the coastline for a several day period," said Gov. Jack Markell of Delaware.
"These factors, along with the potential for power outages, have convinced me that the prudent thing to do is have people leave most of our coastal communities."
Craig Fugate, administrator of FEMA, told Americans: "The time for preparing and talking is about over. People need to be acting now."
Obama assured state governors that FEMA — harshly criticized for being ineffectual during Katrina seven years ago — was ready to help.
"My message to the governors, as well as to the mayors, is anything they need, we will be there," he said.
"And we're going to cut through red tape. We're not going to get bogged down with a lot of rules. We want to make sure that we are anticipating and leaning forward into making sure that we've got the best possible response to what is going to be a big and messy system."
And while both Republicans and Democrats were tiptoeing around politicizing the storm, some progressive news sites pointed out Sunday that Romney called federal disaster relief funding "immoral" in a primary season debate last year.
Instead, Romney said, states should pick up the costs of disaster relief and turn to private contractors if necessary. He also called for the privatization of FEMA.
Americans, meantime, were more concerned with hurricane preparedness than politics on Sunday as they crowded grocery and hardware stores to stock up on food, bottled water, flashlights, batteries and even emergency generators.
Neighbourhoods were bustling with residents bringing lawn furniture indoors, cleaning out their rain gutters, clearing leaves away from storm drains and securing garbage and recycling bins.
"I remember what happened in the summer, and would really rather not go through that again," said Tim French as he pored over generators at a hardware store in Takoma Park, Md.
"I lost about $1,000 in terms of the contents of my fridge and freezer, and the electricity company doesn't reimburse you for that, so I figured spending a few hundred for a generator is a pretty good investment."
Airlines, meantime, cancelled flights across the mid-Atlantic region. Amtrak also announced it was cancelling service across the northeastern U.S. on Monday.