Fred Thompson, the former US Senator, best known as the star of police procedural drama Law Order, chose his appearance last night on the Tonight Show With Jay Leno to announce his candidacy for the Republican nomination for President.

The actor, lawyer and politician, who is a conservative, is in pole position to take the nomination.

Many Republicans see former New York Mayor Rudy Giuliani as too liberal on gay rights and abortion, while Senator John McCain is regarded by many commentators as too old.

Six other candidates, all white men, are running for the Republican nomination.

Mr Thompson has said he is not in favour of a constitutional amendment to ban gay marriage, proposed by President Bush, but would act to ensure the issue remains at state level.

“I know that reform is possible in Washington because I have seen it done. I do not accept it as a fact of life beyond our power to change that the federal government must go on expanding more, taxing more, and spending more forever,” he said in a video posting on his website.

Thompson, 65, is a former Senator from Tennessee and starred opposite Harrison Ford in The Hunt for Red October. He has appeared in over 20 Hollywood movies since 1985.

He has combined entertainment work with law and politics throughout his adult life and his Southern drawl and bulky frame makes him an ideal candidate for what Americans imagine the leader of the free world should look like.

He became a lawyer in 1967 and was co-chief counsel to the Senate Watergate Committee in its investigation of the Watergate scandal from 1973 to 1974.

Mr Thompson represented Tennessee in the Senate from 1994 to 2003.

As well as his recent work on TV police drama Law and Order, he is a well-know radio host in the US.

His candidacy is a particular blow to Senator McCain, who Thompson had backed in 2004.

He has already appeared at fundraising events and is expected to file the required official paperwork in July.

In April he disclosed he has an indolent lymphoma which is not life-threatening.

During a televised debate between some of the Republican hopefuls on Fox TV last night, Rudy Giuliani mocked Mr Thompson’s Presidential aspirations, commenting that he had done a fine job acting a role – as a district attorney in Law Order – that Mr Giuliani had performed in real life.