TAMPA, Fla. — As Tropical Storm Isaac churned into the Gulf of Mexico on Sunday, South Florida managed to dodge the worst of it. But the fast-moving storm appeared poised to swell into a hurricane and hit land on Wednesday between the Florida Panhandle and New Orleans.


Much of the areas along the Gulf Coast, including New Orleans, which was ravaged seven years ago to the week by Hurricane Katrina, received either tropical storm or hurricane warnings on Sunday, and the governors of Louisiana, Mississippi and Alabama declared states of emergency in anticipation of the storm.


The Florida Keys, which were the first land mass in the United States to feel the system’s impact, barely shuddered as the storm sideswiped them with rain and occasional bursts of sharp wind; residents along the chain of islands, long accustomed to storms, were mostly unfazed, while government officials expressed relief.


“It’s not what it could have been,” said Irene Toner, the Monroe County director of emergency management. “I consider us pretty lucky so far.”


Forecasters said the storm could develop into a Category One hurricane — the weakest — by Monday, once it begins its unimpeded journey up the warm waters of the gulf.


Tampa, where most of the formal events on the first day of the Republican National Convention on Monday were canceled because of uncertainty over the storm, will most likely be spared. At the moment, Tampa Bay is expected to feel the sting of the storm’s wind and rain but should escape its most punishing weather, a reprieve to organizers. An estimated 65,000 people are expected here for the convention.


“Tuesday, Wednesday and Thursday ought to be fine,” Mayor Bob Buckhorn of Tampa said on Sunday.


But the storm’s shift to the north and west — farther away from Florida’s south and central coast — has prompted heightened concern elsewhere. Hurricane experts now predict that the storm could smack right into the northern and western Gulf Coast, perhaps even New Orleans. By then, it could reach a Category Two hurricane with 100-mile-per-hour winds, projections showed. The storm’s latest shift, coming three days before the seventh anniversary of Hurricane Katrina, was met with great concern in the region.


Gov. Bobby Jindal of Louisiana said he would skip his scheduled speech in Tampa on Tuesday but might speak on Wednesday if the storm cooperates. The Republican governors in several other Gulf Coast states — Florida, Alabama and Mississippi — also announced that the storm had forced them to alter or drop their plans for the convention.


Mr. Jindal on Sunday asked for voluntary evacuations in 15 low-lying parishes on or near the Gulf Coast and authorized the National Guard to mobilize 4,000 members if needed. The community of Grand Isle was placed under a mandatory evacuation order by its mayor.


“We always have to hope for the best even as we prepare for the worst,” Mr. Jindal said at a news conference in Baton Rouge.


The mayor of New Orleans, Mitch Landrieu, said that he did not anticipate ordering an evacuation but that the city was monitoring the storm and urged residents to be prepared. If an evacuation were to happen, buses and trains would take residents to shelters far from the city; under this plan, the Superdome, convention center and airport would not be open as shelters of last resort.


“We’ve walked through this many, many, many times,” he said. “You have to be prepared to evacuate in the event that an evacuation is called.”


Shell and BP oil companies curtailed drilling and evacuated oil workers in the gulf.


Several Florida beach communities between Tampa and Naples, including Fort Myers Beach, were ordered evacuated on Sunday as a precaution. In South Florida, officials had also taken precautions, closing schools and opening shelters. Airports remained open but hundreds of flights were canceled.


Residents fueled up their cars and generators, took stock of batteries, moved potentially airborne objects inside and bought extra water. In the Keys, most businesses put up shutters and closed early on Sunday. But most people in South Florida took the storm in stride.


As the system shuffled toward the Florida Keys, residents, who tend to view a tropical storm as a trifle, greeted it nonchalantly and, in some quarters, as an excuse to party indoors. They took a few precautions and then met up in bars or with friends. Key West’s last brush with a hurricane was Wilma in 2005, which caused widespread flooding.


With so many people tucked under roofs, raucous Key West felt eerily still.


“The streets are dead,” said Erin Kelly, owner of Bad Boy Burritos in Key West, who planned to stay open until the umpteenth hour. “It’s fabulous.”


Officials had urged tourists to leave ahead of the storm but many in Key West chose to stay put. “It’s a different way to visit Key West,” said Marcello Noccia, a visitor from Naples, Italy. “Everybody here is not nervous. On TV, they are a bit nervous but not here, not walking on the street.”


In Islamorada, farther east along the island chain, many residents were preoccupied with securing their boats. Residents sometimes steer the boats into the mangrove cuts, “then get off the boat and pray,” said Ted Opyr, 68. Mr. Opyr lives on a houseboat and was riding out the storm at the Hampton Inn.


The mangroves can reach about 20 feet in height and are so densely entwined that they provide the best shelter for boats.


As for the locals who did not so much as blink at the storm’s flirtation with the Keys, “You won’t see a local leave the area unless it’s a Category Three or higher,” Mr. Opyr said.