LEWES — Millions of Mid-Atlantic residents made last-minute preparations Saturday — and regional officials warned of devastating flooding and power outages — as Hurricane Sandy continued an ominous trek up the coast and threatened to become the first hurricane to deliver a direct hit on Delaware.

Delaware Gov. Jack Markell on Saturday issued a mandatory evacuation order for more than 50,000 residents living in coastal communities in the state. Markell and leaders in Maryland and the District of Columbia declared states of emergency in anticipation of a megastorm that forecasters said could bring days of heavy rain, sustained wind and tidal surges. Utilities throughout the region warned customers of the prospect of hundreds of thousands of outages that could last for days.

The latest projections call for the storm to reach landfall early Tuesday near the Delaware coast, then collide with two winter weather systems as it moves inland, creating a potentially monster storm.

“You never want to be too naive, but ultimately, it’s not in our hands anyway,” said Andrew Ferencsik, 31, as he purchased plywood and 2-by-4 lumber from a Home Depot in Lewes, Del.

In Maryland, Baltimore Gas and Electric Company warned customers that they should expect to be without power for days. Sustained winds of more than 40 mph were expected to delay workers from restoring outages. The utility, which serves about 1.2 million electric customers in the state, has requested an additional 2,000 out-of-state linemen, tree workers and support personnel. BGE issued a similar warning after powerful windstorms slammed the area late last June, and later came under sharp criticism for its response, which left many households in the dark for the better part of a week.

Though state emergency management officials urged residents of all corners of Maryland to be prepared, the heaviest rain and flooding was expected along the Eastern Shore as Sandy barrels up the coast. One sliver of good news was that the storm was arriving well after tourist season, meaning that far fewer people were likely to be in the worst-hit areas, said Ed McDonough, a spokesman for the Maryland Emergency Management Agency.

“Instead of 300,000 people over there, there may be only a few thousand tourists,” he said. “In fact, with the weather season being what it is, there may not be all that many tourists out there anyway, especially if they saw the weather forecast.”

The Delaware evacuation order was to take place over a 24-hour period starting at 8 p.m. Saturday. Markell declared a limited state of emergency, and said the mandatory evacuation order covered coastal communities in Kent, Sussex and New Castle counties. Those who refused to leave could put themselves and others in harm’s way, he said.

“This is not a police state. People have to take personal responsibility here,” Markell said.

Across Maryland, local and county leaders were urging preparations. The U.S. Naval Academy in Annapolis, a city prone to flooding, was relocating sailboats to a secure location and moving electrical equipment to higher ground.

On the Eastern Shore, Queen Anne’s County was urging residents in low-lying areas prone to flooding to consider evacuating. Harford County has also declared a state of emergency. Ocean City, Md., was urging visitors to the city to postpone visits and said downtown residents should secure their homes and prepare for evacuation.

The Greene Turtle, a popular Ocean City restaurant and bar, didn’t plan to close unless the city forced it to, or unless there were too few workers able to report to their shifts.

“We don’t close up except for Christmas. We try to keep it open as much as we can,” owner Steve Pappas said.

Washington’s Metro system had no immediate plans to change or curtail its service but was urging travelers to sign up for email and text alerts to learn of changes to their rail and bus schedules. Though the train system is generally impervious to rain, sustained wind gusts can lead to speed limits on above-ground rail or cause service interruptions if debris or tree limbs are blown onto the tracks.

The system was prepared to ramp up service if the federal government lets workers out early.

“Our service needs to match that because one-third of our rail ridership is federal government employees,” Metro spokesman Dan Stessel said.

Delaware officials were preparing for the worst blow. In Wilmington, Mayor James Baker issued an evacuation order for the southeastern section of the city. Residents who leave before 6 p.m. on Sunday will be provided with transportation, and police and the National Guard will patrol the evacuated areas to keep them secure, said George Giles, the city’s emergency operations director.

According to the National Oceanic and Atmospheric Administration, some storms have skirted inland after coming ashore further south, and many have passed just offshore, including Gloria in 1985.

“It’s a bit unprecedented, that we are having this type of storm at this time of year. This is the time when most of my guys are on vacation,” said Ian Drummond, a boatyard supervisor at Indian River Marina, south of Rehoboth Beach, who said he and his crews had been working a series of long days to pull boats out of the water and ensure that they were properly tied up