PORT ST. LUCIE, Fla. – Residents and tourists in cars, trucks, and campers clogged highways yesterday in the largest evacuation ever ordered in Florida, fleeing inland as mighty Hurricane Frances threatened the state with its second battering in three weeks.

About 2.5 million residents were told to clear out ahead of what could be the most powerful storm to hit Florida in a decade. Other people in the 300-mile stretch covered by the hurricane warning rushed to fortify their homes with plywood and storm shutters and buy water, gas, and canned food.

Already a Category 4 storm with 140 mph winds and the potential to push ashore waves up to 15 feet high, Frances could make itself felt in the state by midmorning Friday.

At 8 p.m. last night the hurricane was centered 355 miles southeast of West Palm Beach and was moving northwest at close to 10 mph. Hurricane-force winds extended up to 80 miles from its center.

This could be the first time since 1950 that two major storms have hit Florida so close together. On August 13 Hurricane Charley splintered billions of dollars worth of homes, knocked out power to hundreds of thousands, and killed 27 people when it tore across the state.

Charley’s example – and Frances’s tremendous size, at 1,000 miles across, or about as big as the state of Texas – prodded people like Linda Silvestri, 58, to get out of the way. Ms. Silvestri, who lives in Palm Bay on the central Florida coast, headed inland to Gainesville to be near a hospital because she just received a kidney transplant.

“I hope I have a house when I get back,” she said.

The hurricane warning covered most of the state’s eastern coast, from Florida City, near the state’s southern tip, to Flagler Beach, north of Daytona Beach. Forecasters could not say with certainty where Frances would come ashore, just that it would strike late Friday or early Saturday.

About 14.6 million of Florida’s 17 million people live in the areas under hurricane watches and warnings.

Residents and tourists streamed inland in bumper-to-bumper traffic. Traffic backed up for miles on sections of Interstate 95, the main north-south highway along the state’s east coast, and was also heavy on parts of I-4, which crosses the peninsula to connect Daytona Beach, Orlando, and Tampa.

Geoff Connors of Fort Pierce sat in a line of about 50 cars slowly merging onto I-95 in Fort Pierce. He had enough cash and clothes to get through about five days, though he wasn’t sure where he would end up.

“I figured it was smarter to get out of here now. It was a snap decision,” Mr. Connors said.

The storm and the evacuations it forces are certain to spoil Labor Day outings and make a mess of holiday travel across the Southeast.

Erika and Brian Marwood, who moved to Orlando from Colorado two months ago and huddled in their bathroom with glow sticks and candles while Charley rushed overhead, made their way this time to a Holiday Inn in Tifton, Ga.

“We thought we were doing a good thing getting away from the snow, but there are no hurricanes in Colorado,” Erika Marwood said.

Governor Bush asked his brother President Bush to declare Florida a federal disaster area and make storm victims eligible for recovery aid.

Federal officials promised they had enough people and supplies in the state to handle two disaster-relief operations at once.

“We were successful with Charley because we were massive, overwhelming, and fast. For this event I want us to be massive, overwhelming, and fast squared,” said the director of the Federal Emergency Management Agency, Michael Brown.

People flocked to airports, hoping to get out before all flights were grounded. Some trudged through long lines at ticket counters only to find their flights had been canceled. Hotels and motels inland filled up, and gas stations ran dry.

Florida rescinded tolls on major roads and said lanes on some highways may be reversed to handle the evacuation traffic. State officials hoped to avoid a repeat of the mess during Hurricane Floyd in 1999, when 1.3 million people were told to evacuate the state’s east coast and traffic backed up 30 miles or more.

The Kennedy Space Center at Cape Canaveral was ordered completely evacuated for the first time because of the dual threats of high wind and storm surge.

Many businesses along the Atlantic coast began closing Wednesday. Stores that were open were stripped of bottled water and canned goods, and long lines formed outside home supply stores as people hoped for a chance to buy scarce plywood or generators. The arrival of a delivery truck was met with raucous applause in Palm Beach County.

Jenny Stimpson, 32, joined hundreds of others at a Wal-Mart in Stuart hunting for last-minute supplies, but could find only ice. She said she bought 25 bags because “everywhere you go, you better grab something because it won’t be there if you go back later.”

Frances is as strong as Charley, and twice its size, said Stephen Baig, a forecaster at the National Hurricane Center in Miami. Frances is also about twice the size of 1992’s Hurricane Andrew, the Category 5 storm that destroyed much of southern Miami-Dade County.

The last time two major storms hit Florida so close together was 54 years ago, when Hurricane Easy hit the Tampa area and Hurricane King struck Miami about six weeks later. Neither storm was as powerful as Charley or Frances.