Fred Thompson, the former senator and Law and Order star, quit the Republican presidential race last night but left open the question of whether he will endorse his old friend John McCain – a move that could reshape the wide-open contest and impact on the next showdown in Florida.

Mr Thompson dropped out after a string of poor finishes in the early primary and caucus states, a result of one of the most bafflingly lackadaisical and listless campaign efforts of recent times.

He entered the race late but came to it amid huge anticipation and hope among conservatives that he would save the party from a field of candidates with no true conservative standard-bearer. Some even talked of him as the heir to Ronald Reagan. He had perhaps the greatest name recognition of all the candidates, after a highly successful career on television and in Hollywood, in which on one occasion he played a US president.

Yet he also entered the race with a reputation for being workshy and lazy, a perception that he only reinforced from the moment that he declared his White House ambitions. After an opening weekend in Iowa and New Hampshire, he failed to return to those states for three weeks. When he did appear on the stump, he seemed at times disinterested and lacking the fire in the belly needed to win the nomination.

His fate was sealed on Saturday in the South Carolina primary, when he finished third in a state that he said that he needed to win. By the time of the vote his campaign was already drastically short of money.

“Today I have withdrawn my candidacy for President of the United States.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.” He did not say if he was going to endorse a candidate, despite widespread speculation in recent days that he would back Mr McCain. He was one of a handful of members of Congress who supported Mr McCain in his unsuccessful 2000 primary race against George Bush.

Even if Mr Thompson does back Mr McCain, his withdrawal also helps another fellow Republican: Mike Huckabee. Both men draw from the same pool of evangelical voters and there is a belief among Mr Huckabee’s aides that he would have won South Carolina, rather than coming a close second to Mr McCain, if Mr Thompson had dropped out earlier.

As the remaining candidates tear through Florida before next week’s primary – in one of America’s most diverse swing states – they are spread from Little Havana in the south to the Baptist heartland in the Panhandle north, the largest and most complex battlefield that they have faced and a contest that could dramatically winnow the Republican field. If Rudy Giuliani, the former New York Mayor, fails to win on January 29, the Republican race will likely boil down to a contest between Mr McCain and Mitt Romney.

The muddled Republican race enters a crucial new phase in Florida. It is a winner-takes-all battle for its 57 delegates. It is the first primary in which Mr Giuliani has competed at full force, the first truly Republican contest closed to independent voters, and the first test of a candidate’s ability to appeal to a sprawling, multicultural electorate in a state of 18 million people, making it the gateway to February 5, Super Tuesday, when 22 states vote.

Each leading candidate has his own core constituency: Mr McCain, military families; Mr Romney, the business community; Mr Giuliani, the huge number of retired New Yorkers, many of whom are Jewish; and Mr Huckabee, evangelicals. Mr McCain campaigned in Little Havana in Miami on Monday, while Mr Romney and Mr Giuliani will speak to the Latin Builders Association on Friday. But they must also all target its key demographic: voters over 55, who account for three quarters of Florida’s Republican electorate, as well as the fiercely antiCastro Cuban-Americans, who dominate the Miami area.

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