Hurricane Sandy remained a threat to overwhelm vast portions of the East Coast on Saturday, even as experts said it had weakened as its center reached 350 miles southeast of Charleston, S.C.

As people from North Carolina to Cape Cod scrambled to prepare, Sandy’s speed had declined to 75 miles per hour, making it possible that it would lose its status as a tropical storm or hurricane.

At 8 a.m. Sunday, the storm was about 260 miles southeast of Cape Hatteras, N.C., moving at about 10 mph, the National Hurricane Center reported.

Meteorologists said Sandy could still uncoil with the force of a hurricane, as it merges with a jet stream and a nor’easter, triggering what is likely to be several days of destruction on its northern path through eight states.

Two weather forecasting models have agreed that the storm likely will reach shore somewhere between the Delmarva Peninsula and Rhode Island. And it is no longer a question of whether the Washington region will get hit, but of how badly.

Several factors combine to make this storm particularly dangerous.

“At this point there are essentially two scenarios for the area: a worst case scenario (less likely) and a very bad scenario (more likely) — but the differences between the scenarios are not significant for most of us,” according to Jason Samenow, meteorologist with The Washington Post’s Capital Weather Gang.

Samenow said the worst of the storm is expected to hit Monday into Tuesday, bring 3 to 6 inches of rain and winds of 25 to 45 mph. If the storm makes landfall north of the District, there could be major coastal flooding for Maryland and Delaware beaches but less damaging winds. If the region takes a direct hit, Samenow said “historic coastal flooding” could occur along the beaches. The region could also see hurricane force wind gusts over a 48 hour period, he said.

As Saturday’s bright sun and warm temperatures made the prospect of an epic storm more difficult to imagine, rain was still forecast for Sunday, a lead up for Sandy’s arrival.

The governors of Maryland and Virginia already have declared states of emergency, as has District Mayor Vincent Gray. Flood watches were issued in the District, as well as in Anne Arundel, Frederick, Howard, Montgomery, Prince George’s, Calvert, Charles, St. Mary’s, Arlington and Falls Church counties. A coastal flood watch was issued for all of Maryland’s beaches, as well as Virginia Beach.

Wilmington, Del., Mayor James M. Baker has issued a mandatory evacuation of the southeastern section of the city and has declared a state of emergency. The evacuation order applies to about 3,000 residents who live in a flood plain area of the city. Baker said Saturday he wants to give residents affected by the order time to make arrangements to stay with family and friends. Transportation will be provided until 6 p.m. Sunday for those who need it.

Ocean City, Md. officials asked those who planned visits to the beach to postpone them. The city is expecting at least 10 inches of rain and flooding in low-lying areas.

The region’s utility providers called on companies outside the area to send as much help as possible. Dominion Virginia Power asked to borrow 2,000 workers, while Pepco asked for 2,500 to be sent to the District and Maryland. Pepco warned that hundreds of thousands of customers may lose power if the storm hits the area as expected.

Across the area, people stocked up on food, gas and other supplies, if they could find them.

David Kline, 41, filled up two gas cans at an Exxon just outside Ocean City for a hard-to-find generator he had just purchased.

“[Hurricane] Irene … that was kind of like a dud,” he said of last year’s big storm that led officials to put in place a mandatory evacuation of Ocean City. This, he is convinced, will be more serious.

In the District, the flashlight section at the Target in Columbia Heights was bare.

“Everyone has freaked out. Wow,” said Jennifer Gaskins, 59, who drove from her home in Brightwood.

Up and down the Chesapeake Bay, owners were trying to secure their boats, and those who could were pulling them out of the water. Greg Wilson, of Grasonville, towed away his fishing boat after receiving a warning “saying we should haul out if we could.”

“I can always bring it back to go fishing next weekend,” he said.

Also preparing for a storm were mass-transit systems, airports and airlines.

Megabus announced they were canceling service between New York and Washington after 5 p.m. Sunday. Amtrak officials tweeted that they would be announcing changes to their service soon.

The three major commuter systems in the D.C. area — Metro, the Maryland Transit Administration and Virginia Railway Express — plan to continue normal operations until conditions require otherwise.

“If conditions really deteriorate on Sunday, we may change that, but at this point, we’re proceeding,” Metro spokesman Dan Stessel said.

The transit agency began placing sandbags around vent shafts and station entrances of the subway system. Crews had also begun checking pumping stations that will funnel water out of the stations, ensuring that drains were clear and that enough chain saws were available if there were downed trees. Also, additional rail and bus supervisors are being called in to work at control centers.

The MTA began mobilizing emergency operations and maintenance workers, said spokesman Joe Sviatko.

“As long as it’s safe, the goal is to keep everything open that we can during our normal operational hours,” Sviatko said.

The agency has additional diesel trains available for MARC’s Penn Line, which typically uses electric trains, to try to ensure that there is MARC service on Monday. MTA officials will assess the storm’s path Sunday afternoon to determine the extent of service available on the system’s buses, trains and light-rail system.

VRE is planning full service on Monday, barring any major weather-related problems.

Staff writers Jeremy Borden, Ashley Halsey and Coleen O’Lear contributed to this story.