WASHINGTON (AP) — President Barack Obama delivered a sober warning to millions in the path of Hurricane Sandy on Monday, declaring that even though food, water and generators have been moved into position, “this is going to be a difficult storm” with long-term power and transportation outages possible.

In a direct appeal to those who live in harm’s way, the president said, “Please listen to what your state and local officials are saying. When they tell you to evacuate, you need to evacuate. Don’t delay, don’t pause, don’t question the instructions that are being given because this is a powerful storm.”

The president made his remarks in the White House briefing room eight days before the Nov. 6 election, and a few hours after he canceled a campaign appearance in Florida to return to Washington. He turned aside a question about the storm’s impact on the campaign, saying safety was his top priority.

The huge storm posed a threat to an estimated 50 million people and some of the nation’s most densely populated areas. Even before a nighttime projected landfall along the mid-Atlantic Coast, dire warnings of winds, rain and storm surges prompted officials to close mass transit systems in New York, Boston and Washington, as well as Connecticut’s highways. The federal bureaucracy in Washington also was shuttered.

Obama said he had spoken with the governors of all the states likely to be affected, and he added there had been “extraordinarily close coordination” among various levels of government.

Yet he stressed repeatedly the dangers posed by the slow-moving storm, and said its effects would not dissipate quickly.

“The public should anticipate that there are going to be a lot of power outages,” he said.

He added, “Transportation is going to be tied up for a long time. … We anticipate that there are going to be a lot of trees down, a lot of water.”

The president said, “We’re making sure that food and water and emergency generation are available for communities” that need them.

Obama had been scheduled to make a campaign appearance in Florida before flying back to the nation’s capital, but he scrapped those plans in favor of an immediate, bumpy flight aboard Air Force One.