Thompson's statement from his McLean, Va., home said simply:

"Today I have withdrawn my candidacy for President of the United States. I hope that my country and my party have benefited from our having made this effort. Jeri (his wife) and I will always be grateful for the encouragement and friendship of so many wonderful people."

Thompson spokesman Jeff Sadosky said the campaign had nothing to add.

The withdrawal a week before the Florida primary came as no surprise, given Thompson's third-place showing in Saturday's South Carolina primary. He had staked his White House hopes on doing well in the first Southern state primary and finished with 16 percent of the vote, far behind McCain and ex-Arkansas Gov. Mike Huckabee.

While Thompson's campaign was not successful, he could still have an impact on who receives the nomination. Thompson could encourage his social conservative supporters to back McCain.

Huckabee has blamed Thompson for his second-place showing in South Carolina, claiming Thompson stayed in the race to siphon off social conservatives from him.

Thompson's withdrawal will likely help Huckabee in the South, said Bruce Oppenheimer, political science professor at Vanderbilt University.