By Nandita Bose and Trevor Hunnicutt HALLANDALE BEACH, Fla. (Reuters) -U.S. President Joe Biden warned Florida voters that Social Security and Medicare could be repealed by Republicans, an effort to make healthcare and retirement benefits an issue ahead of elections determining whether Democrats hold Congress. “You’ve been paying into Social Security your whole life, you earned it, now these guys want to take it away,” Biden said in a speech describing the programs as “under siege” by Republicans. “Who in the hell do they think they are?” The speech on Tuesday at a sparsely attended South Flor…