Emergency personnel in Florida rushed yesterday to finalise contingency plans to postpone or relocate next week's Republican Party convention in Tampa, should the area be lashed by Tropical Storm Isaac.
As it tracks into the Gulf of Mexico, warm waters could pump it up to hurricane strength.
As of last night, the gathering that will draw 50,000 to Tampa was due to go ahead.
It is a key moment for Mitt Romney, who hopes to use his position as the Republican presidential nominee as a stage to introduce himself to American voters.
Predicting the path and the likely intensity of the storm remained precarious. Forecasters placed Tampa in a so-called "cone of uncertainty", meaning it could take a hit from the storm. But most models had it tracking to the west of the city, making landfall along the Florida Panhandle or New Orleans.
The most dangerous time appears to be Monday evening, precisely when proceedings in the Tampa Bay Forum, a hockey arena, are due to kick off.
Any attempt to alter the convention plans could create pandemonium among participants staying in hotels across the region. The Republican spokesman Sean Spicer said organisers were prepared for everything.