Former crime series actor Fred Thompson pulled out from the Republican nomination race after his campaign as a "true conservative" failed to catch fire. He served in the US Senate for more than eight years until 2003 and, as a young lawyer was on the Republican staff of congressional investigations into the Watergate scandal in the early 1970s.

Ex-senator from Tennessee, Fred Thompson is also an actor, best known for his role as Arthur Branch, the New York City District Attorney in the long-running television series Law & Order, aired by NBC. He announced his candidature last year in September.

Thompson bowed out after finishing a distant third in Saturday's Republican primary in South Carolina behind Senator John McCain and former Arkansas governor Mike Huckabee, winning just 16 per cent of the vote.

"Today I have withdrawn my candidacy for president of the United States," Thompson said on his website. "I hope that my country and my party have benefited from our having made this effort."

"I hope that my country and my party have benefited from our having made this effort. [My wife] Jeri and I will always be grateful for the encouragement and friendship of so many wonderful people," he added.

Some supporters compared the former lawyer and to Ronald Reagan, the Hollywood actor who became US president and a conservative icon in the 1980s. But Thompson never generated the money or momentum to match, while media described his campaign as lacklustre.

He failed to win any of the six state preference polls held so far by the Republican Party to choose its candidate for the November 4 presidential election. As a Southern state like his home state of Tennessee, South Carolina had been viewed as a make-or-break contest for Thompson.