* Tropical storm warning in effect for south Florida coast
* Isaac threatens landslides, flooding in Haiti
* Hundreds of thousands of Haiti quake survivors still in tent camps
Tropical Storm Isaac strengthened as it dumped heavy rains on Haiti on Saturday, threatening floods and mudslides in a country where hundreds of thousands of people remain homeless more than two years after a devastating earthquake.
Lashing rains and high winds were reported along parts of Haiti's southern coast and in the Haitian capital of Port-au-Prince, where more than 350,000 survivors of the 2010 earthquake are still living in fragile tent and tarpaulin camps.
Intermittent power outages affected the greater Port-au-Prince area in the early hours of Saturday as Isaac bore down on the impoverished Caribbean country.
Isaac was about 65 miles (100 km) south-southwest of Port-au-Prince late Friday night, the U.S. National Hurricane Center (NHC) said.
The storm had sustained winds of 70 miles per hour (110 km per hour) and its center was expected to pass over Haiti's southern coast early Saturday.
Isaac's march across the Caribbean comes as U.S. Republicans prepare to gather in Tampa, on Florida's central Gulf Coast, for Monday's start of their national convention ahead of the November presidential election.
The convention is still expected to proceed as planned but Gulf of Mexico operators began shutting down offshore oil and gas rigs on Friday ahead of the storm.
But the biggest immediate concern was heavily deforested Haiti and the NHC warned there was a possibility Isaac could reach hurricane intensity before making landfall in Haiti.
On Friday, the government and aid groups evacuated thousands of tent camp dwellers but many Haitians chose to remain in their flimsy, makeshift homes, apparently out of fear they will be robbed, said Bradley Mellicker, head of disaster management for the International Organization for Migration (IOM).
"There's a lot of people who are resisting because they are scared of losing what little they have now," Mellicker said.
About 3,000 volunteers from the government's Civil Protection office were dispatched across Haiti, warning people about flood and landslide risks, and about 1,250 shelters -- schools, churches or other community buildings -- that have opened their doors to house people seeking refuge from the storm.
But Red Cross officials said the number of shelters could be grossly inadequate and Prime Minister Laurent Lamothe acknowledged Haiti had "limited means" to ensure public safety.
Red Cross and IOM representatives joined government officials in trying to evacuate 8,000 of the "most vulnerable people," including 2,500 sick and disabled, from 18 tent camps in low-lying coastal areas of Port-au-Prince.
Many Haitians, most of whom scrape by on less than $1 per day, consider disaster an inevitable part of life in the poorest country in the Americas.
"We live under tents. If there's too much rain and wind, water comes in. There's nothing we can do," said Nicholas Absolouis, an unemployed 34-year-old mechanic at one camp for homeless people on the northern edge of the chaotic capital.
"There are still too many people living in the camps. There's a good chance that those might be destroyed with the passage of the cyclone," said France Hurtubise of the International Federation of Red Cross and Red Crescent Societies in Haiti.
Flooding could also help reignite a cholera epidemic, which has killed more than 7,500 people in Haiti since the disease first appeared in October 2010, foreign aid workers said.
On its current path, forecasters said Isaac would hit Cuba and the southern tip of Florida before strengthening into a Category 1 hurricane over the Gulf of Mexico and making landfall anywhere from the Florida Panhandle in the northwestern part of the state to Alabama and as far west as New Orleans.
A tropical storm warning was issued for the entire coast of south Florida on Friday, and a hurricane warning also went into effect in the Florida Keys.
Isaac has drawn especially close scrutiny because of the Republican Party's convention, a four-day meeting during which former Massachusetts Governor Mitt Romney will receive the party's presidential nomination.
Party officials insist the convention will go ahead, even if they have to alter the schedule. But NHC meteorologist Rick Danielson said Tampa could potentially be hit by coastal flooding and driving winds or rain.
"There is still a full range of possible impacts on Tampa at this point," he said.
Danielson said it was very hard to project intensity before Isaac passes over mountainous Cuba on Saturday and Sunday and enters the Florida Straits. But the Florida Keys, the island chain off the southernmost part of the state, were definitely in harm's way.