President Barack Obama on Monday urged East Coast residents in the path of Superstorm Sandy to heed evacuation orders and assured them the government was ready to respond swiftly, but he warned them it would take a long time to clean up in the storm's aftermath.

As Americans braced for the storm, New York City evacuated neighbors of a partially built, 90-story apartment building after the top of a construction crane collapsed in high winds, prompting fears the entire rig could crash to the ground. The upper arm of the crane dangled over the street near Central Park from what should eventually become the city's tallest residential building.

Broadway theaters were closed ahead of the storm, and Republican Massachusetts Senator Scott Brown and Democratic challenger Elizabeth Warren to scrap plans for their fourth and final debate scheduled for Tuesday night.

Some 2.2 million Americans were without power in 11 states, as well as in the D.C. area, and 23 states were under high warning or advisory as the storm was coming ashore, CNN reported.

Scrapping campaign plans to return to Washington, Obama sought to show voters just eight days before the November 6 election that he was giving top priority to his presidential duties in a looming national crisis, rather than his bid for re-election in a tight race.

He also appeared determined to demonstrate that his administration had learned the lessons of White House predecessor George W. Bush's botched handling of Hurricane Katrina in 2005, which inflicted serious political damage.

Rushing back from a campaign visit to Florida, Obama huddled with top aides in the White House Situation Room for an update on Sandy, which started to batter the densely populated East Coast as one of the biggest storms to ever hit the U.S. mainland.

It was forecast to move ashore Monday night in New Jersey as what Obama called a "big and powerful storm" whose slow-moving course would affect millions of people.

With Sandy expected to bring massive flooding, power outages and other disruptions all along the Atlantic coast, Obama seemed mindful of the political risk of disgruntled storm victims on the cusp of the election, and he appealed in advance for patience.

"I'm confident that we're ready, but I think the public needs to prepare for the fact that this is going to take a long time for us to clean up," he said. "The good news is we will clean up and we will get through this."

Obama has tried to draw a sharp contrast with the Bush administration, which was heavily criticized for its slow and inept handling of Katrina as the hurricane devastated New Orleans early in his second term.

He has also sought to project the image of a president fully engaged in marshaling resources to deal with a looming national emergency. Bush was widely seen as out-of-touch during the Katrina crisis.

Obama faces political danger if the federal government's response goes awry in the final days of the election campaign, but he also has a chance to look presidential compared with his Republican challenger, Mitt Romney.

Obama had blunt words for those in Sandy's path.

"When they tell you to evacuate, you need to evacuate. Do not delay. Don't pause," he said. "Don't question the instructions that are being given, because this is a serious storm and it could potentially have fatal consequences."

Already looking to the problems likely to crop up after the storm that could become an issue before Election Day, Obama said:.

"The public should anticipate that there's going to be a lot of power outages and it may take time for that power to get back on.

"The same is true with transportation; there are going to be a lot of backlogs. And even after the storm has cleared, it's going to take a considerable amount of time for airlines, subways, trains, and so forth, potentially, to get back, you know, on schedule."

However, the U.S. president said, "the election will take care of itself next week."

"Right now our number one priority is to make sure that we are saving lives, that our search-and-rescue teams are going to be in place, that people are going to get the food, the water, the shelter that they need in case of emergency, and that we respond as quickly as possible to get the economy back on track," he added.