The storm, with top sustained winds of 40 mph was projected to become a hurricane Thursday, the hurricane center said.
Isaac was on a path suggesting it will batter the Tampa Bay area on Florida's west central coast along the Gulf of Mexico sometime in the mid-afternoon Monday, the map of the storm center's "probable path" indicated.
Forecasters said it was still too early to know for sure.
"There is still way too much uncertainty right now, and it's too early to know what effect it could have in the U.S. and in Florida," center spokesman Dennis Feltgen told The Washington Post.
"It depends on how much time it spends on land in Cuba," he said. "We don't know what kind of shape it will be in by the time it clears those islands."
Isaac is forecast to hit Cuba Saturday and Sunday where the U.S. military at Guantanamo Bay Naval Base was forced to postpone hearings for five alleged plotters of the Sept. 11, 2001, terrorist attacks.
Base commanders have made emergency hurricane preparations.
Hurricane warnings were in effect for all of Haiti and parts of the adjoining Dominican Republic, southeast of Cuba.
Even though the hurricane center in Miami said Isaac's path was not certain, forecasters urged "folks to pay attention to this thing," Feltgen told the Post.
Tampa Mayor Bob Buckhorn told CNN one of the options being considered was canceling the GOP convention, scheduled to run Monday through Thursday.
"Absolutely, we're prepared to call it off," he said.
Buckhorn, a Democrat, later clarified to The Wall Street Journal that such a move "would be a last resort."
"I don't think it will come to that," he said. "We are full speed ahead with the RNC."
Convention Communications Director James Davis told the Journal contingency plans were "in place to ensure the health and safety" of attendees and residents, but declined to elaborate.
The U.S. Secret Service coordinates overall convention security and would involve many federal, state and local agencies in a disaster response, spokesman George Ogilvie told the Journal.
RNC planners canceled some events on the opening day of the party's 2008 convention in St. Paul, Minn., out of respect for people threatened by Hurricane Gustav, the second-most destructive hurricane that year, which was bearing down on the Gulf Coast.
Bush spoke the next day from Washington by satellite. Cheney did not address the convention.
The major U.S. broadcast networks plan to cut back on their coverage of the convention -- including at 10:30 p.m. EDT the opening day, when Ann Romney, wife of presumptive Republican presidential nominee Mitt Romney, is scheduled to deliver a keynote address.
CBS plans instead to show a "Hawaii Five-O" rerun, NBC has scheduled a new episode of "Grimm" and ABC has slotted "Castle."
The campaign is considering the possibility of moving her speech to another night, The New York Times and Politico reported.
The networks told the Romney campaign they intend to broadcast an hour of convention coverage on the four-day convention's final three nights -- and leave the gavel-to-gavel coverage to cable news.
They plan to do the essentially same thing for the Democratic National Convention, set for Charlotte, N.C., Sept. 4-6, right after Labor Day.
The only difference is that NBC plans to skip Sept. 5 coverage altogether in favor of the opening game of the regular NFL season. Defending Super Bowl XLVI champion New York Giants is to host the Dallas Cowboys.