MIAMI – Nearly a half-million people were ordered to evacuate as Hurricane Frances swirled toward Florida yesterday just weeks after Charley’s devastating visit, threatening to deliver the most powerful one-two punch to hit a state in at least a century.

Those planning to ride out the storm snapped up canned food, water, and generators, while military helicopters and planes were flown out of the area and Cape Canaveral’s Kennedy Space Center said it would close today.

Forecasters said the still-strengthening Category 4 storm could hit on Labor Day weekend as early as Friday night, less than three weeks after Charley raked Florida’s western coast with 145 mph winds, causing billions of dollars in damage and killing 27 people.

“I can’t emphasize enough how powerful this is. If there’s something out there that’s going to weaken it, we haven’t seen it,” National Hurricane Center director Max Mayfield said.

Governor Bush and Governor Perdue of Georgia declared states of emergency, activating the National Guard. Mr. Bush also warned more evacuations may be ordered.

Many homes in southwest Florida still have blue tarps patching holes in their roofs from Charley, and some streets remain full of storm debris that could become wind-blown projectiles.

“We’ve just gone through 2 1 / 2 weeks of torture trying to get our lives back to some sense of order,” said a Punta Gorda retiree, Tom Hamilton.

Evacuation orders were posted for 300,000 residents in coastal areas of Palm Beach County, and nearly 200,000 were told to leave mobile homes and low-lying areas of Brevard, Martin, and Indian River counties, which could be hit by tremendous ocean waves. The evacuation orders were set to take effect this afternoon.

Frances was about 650 miles southeast of Florida yesterday evening, heading northwest on a course that would take it to the central portion of Florida’s eastern coast. Residents of Georgia, North Carolina, and South Carolina watched the forecast closely in case Frances took a sharper turn to the north.

Mr. Bush cautioned that “all the science in the world and all the technology in the world isn’t going to be able to pinpoint exactly where the storm goes.”

Frances would be the fourth storm to affect Charleston, S.C., this summer. Bonnie and Charley arrived within days of each other in August, and Gaston dumped more than 13 inches of rain in some areas when it came ashore Sunday.

With the ground saturated from previous storms, more rain and the slightest wind could cut utility service for thousands, even if they don’t take a direct hit from the storm. “With the ground this wet, trees could easily topple,” said Tom Williams, a spokesman for Duke Power in Charlotte, N.C.

The storm and the mass evacuations are sure to spoil Labor Day outings and make a mess of holiday travel across the Southeast. Florida may reverse lanes on some highways to handle the evacuation traffic, state Emergency Management Director Craig Fugate said.

If Frances hits, it could be the most potent two-hurricane combination to hit a single state in at least a century of record-keeping. Frances is as strong as Charley, but forecasters said it could become a Category 5 with winds of 156 mph or higher.

Hurricane-force winds extended about 80 miles from Frances’s center, making it about twice the width of Charley and increasing the possibility for damage.

The last time two major hurricanes hit Florida in rapid succession was 1950. Hurricane Easy struck Tampa around Sept. 4 of that year and Hurricane King hit Miami six weeks later on October 17. They were Category 3 storms.

Nancy Cuffaro of Port Charlotte, whose home and pizza restaurant were damaged by Charley, said she is hoping Frances spares an area that is still suffering. “I know we can’t withstand too much. I really don’t know what to think here. I’m lost. It’s starting to get to me,” she said.

A Home Depot to the south in Florida City, hit hard by Hurricane Andrew 12 years ago, more than doubled its daily sales and ran out of generators and plywood. It sold $50,000 worth of lumber Tuesday, said assistant manager Lisa Ftiffler.

North Miami Beach resident Lorraine Lewis bought a small cooking stove in case of emergency but wasn’t planning to stick around long enough to use it. “I have water and plastic and a plane ticket,” she said.

State officials worried about finding enough room in shelters. Many hotel rooms in southern Florida are occupied by emergency workers and people left homeless by Charley. Some community centers are still being used as shelters.

Deanna Creamer, who rode out Charley in her house in Punta Gorda, was still repairing roof and water damage. “I heard a little bit on the radio this morning, and I shut it off,” said Ms. Creamer, 40. “I just can’t imagine having to go through this again.”