The new threat on Tuesday comes three weeks after Hurricane Katrina swamped New Orleans and hammered the US Gulf Coast.
The state Health Department on Tuesday raised the death toll from Katrina in Louisiana by 90 to 736. The toll across the Gulf Coast was 973.
All 80,000 residents were ordered out of the Keys on Monday, and Miami-Dade mayor Carols Alavrez cautioned southern Florida not to dismiss the power of the coming storm.
"Tropical Storm Rita is a serious threat. Do not underestimate this storm," he said. "Stay home. No matter what, we're going to have lousy weather." Schools, many government offices and some businesses were closed on Tuesday.
A Louisiana official warned that levees or embankments in New Orleans, where hundreds died in Katrina's floods, would fail again if the city were smashed by a new storm surge, and the city ordered residents to leave.
Forecasters say Rita will
likely reach hurricane strength
Some residents had been allowed back into the city to assess the damage to their homes and were being told to leave again.
Oil companies starting to recover from Katrina began to evacuate Gulf oil rigs.
Private forecasters said there was a 40% chance that damaging hurricane-force winds would directly affect major Gulf energy production areas.
Rita was expected to become a major hurricane on Tuesday with sustained winds of at least 178kph as it drew strength from warm Gulf waters after passing over or near the Florida Keys on Tuesday, the US National Hurricane Centre in Miami said.
Forecasters said Rita, the 17th tropical storm of an exceptionally busy Atlantic hurricane season, would likely reach hurricane strength, with winds of 119kph or greater, early on Tuesday. Its sustained winds were 110kph.
The Hurricane Centre cautioned that Rita could still veer north to the Miami area, home to 2.3 million people.
New Orleans mayor Ray Nagin on Monday suspended a plan to bring residents back, and told all those now in the stricken city to leave because of fears that Rita could swamp damaged levees and wreak new havoc.
But "now we have conditions that have changed. We have another hurricane that is approaching us," Nagin said. He warned that the city's pumping system was not yet running at full capacity and that the levees were still in a "very weak position".
He ordered residents who circumvented checkpoints and slipped back into the still officially closed parts of the city to leave immediately.
The Atlantic hurricane season runs from June to November and produces an average of about 11 tropical storms or hurricanes.
Forecasters had predicted an unusual 2005 season with up to 21 storms due to warm sea temperatures and other conditions favourable to hurricanes.