Senator John McCain won the Republican primary in Florida, putting him in pole position to become the party's presidential candidate in November.
Ex-Massachusetts Governor Mitt Romney came in a close second behind Mr McCain.
Ex-New York Mayor Rudy Giuliani, the former Republican front-runner, trailed in third place, leading experts to predict that he would withdraw from the race and endorse Mr McCain.
Ex-Arkansas Governor Mike Huckabee vowed to fight on, after taking fourth place in the contest.
Senator Hillary Clinton won the state's Democratic election, although the poll will not count towards the nomination after a party row over scheduling.
Election officials announced a high turnout in both parties' contests, with some three million voters taking part, more than in 2004 and 2000.