Tropical Storm Isaac is posing a potential threat to next week’s Republican National Convention in Florida, which culminates in the nomination of Mitt Romney for president. The U.S. National Hurricane Center on Wednesday morning said Isaac was expected to strengthen and could become a hurricane by Thursday. Convention organizers knew it was a possibility during the peak of hurricane season. About 70,000 delegates, party officials, journalists, protesters and others are expected to attend. It’s been 90 years since a major hurricane made a direct hit on Tampa, the site of the convention. Florida, historically the nation’s top target for tropical systems, has not been hit by a major hurricane since Wilma in 2005.