THE entry of Fred Thompson, star of the US television series Law & Order, into the race for the 2008 Republican nomination has threatened to destroy the presidential hopes of Senator John McCain, his best friend in politics.

Tony Fabrizio, a Republican consultant, said: “If I were McCain, I would have certainly had a conversation with Fred Thompson and said, ‘What are you doing?’ ” Thompson, the former senator for Tennessee, served as co-chairman of McCain’s 2000 presidential campaign. He said then that his friend, a Vietnam war hero, showed “characteristics of leadership like nobody I’ve ever seen”. They sat next to each other in the Senate and Thompson was one of a handful of Republican senators who backed McCain’s campaign finance reform, a controversial law much resented by Republican activists.

Larry Sabato, a political analyst at the University of Virginia, said it was a case of “Et tu, Brute, Fred”, a reference to the last words of Julius Caesar (according to Shakespeare) when he was stabbed to death: “I’m sure he is very unhappy to be knifed by his friend.” Until the start of the year Thompson was quietly raising money for McCain’s 2008 campaign. When the rumour spread that he might join the race himself, there was talk that he would serve as a stalking horse who would ultimately direct his supporters to back McCain.

That theory was scotched when Thompson formed a “Friends of Fred Thompson” committee on Friday. He is likely to announce formally that he is running in the week of July 4, Independence Day.

He will immediately join the top tier of Republican candidates. He has “Ronald Reagan-like appeal”, said Katon Dawson, chairman of the Republicans in South Carolina, a key primary state. “He has great personal charisma and the ability to convey a conservative vision for America.”

McCain last week welcomed Thompson into the race. “Fred’s a very good friend,” he said. “I guess my words are: come in, the water is fine.” But when he was asked if the actor could affect his chances, he said soberly: “I don’t know.”

The Arizona senator had been devoting many of his resources to South Carolina since falling foul of social conservatives there in 2000 and had been rewarded with modest polling success.

Thompson’s southern “good ol’ boy” roots and movie star credentials – in Hollywood films he has played the president, the head of the CIA, the White House chief of staff and an admiral – could siphon off support and money from his friend, who is struggling with fundraising.

McCain’s support for the controversial plans of President George W Bush for immigration reform has set him further at odds with an incandescent Republican party base. He had already lost the backing of many independents over his defence of the troop “surge” in Iraq.

“McCain is in danger of becoming a man without an island,” said Fabrizio. “If you can’t be the guy who brings in the moderates and independents and you can’t win the conservative vote, who are you?”

Thompson’s likely candidacy is also a blow to Mitt Romney, former governor of Massachu-setts, who has tacked to the right in the campaign to fill the gap left by the absence to date of a strong social conservative.

Although support for Rudy Giuliani, the frontrunner, is expected to falter, insiders predict that the socially liberal, crime-busting hero of the September 11 terrorist attacks will emerge as the last man standing against the candidate of the right, who may well turn out to be Thompson.

The pressure is on the actor to demonstrate the magnetism that his supporters have been claiming for him. Thompson, 64, is expected to dust off the red pickup truck that has been sitting on his mother’s drive since he first ran for the Senate in 1994. Legend has it that Thompson’s real car was a luxury silver sedan, but the pickup confirmed his folksy appeal.

He intends to run as a Washington outsider, despite years in the city as a lawyer during the Watergate hearings, when he uncovered the existence of the Nixon tapes, and later as a lobbyist.

In 2002 Thompson married Jeri Kehn, 40, a Republican political consultant nearly 25 years his junior. The New York Post last week described the mother of two children aged four and seven months as a hot “babe”. She had been involved in a “cat-fight” for Thompson’s affections with a journalist in Washington, but was also a shrewd adviser.

It was Jeri who advised Thompson on his greatest campaign coup: his fast video response to a provocation by Michael Moore, the film-maker, who had accused him of violating the trade ban with Cuba by smoking a Havana cigar.

Puffing away, Thompson suggested that Moore take up the cause of a dissident documentary film-maker who was locked up in a Cuban mental institute by Castro’s regime.

McCain’s Straight Talk Express campaign bus seems dated by comparison and the 70-year-old senator is battling doubts about his age. Thompson also has his demons: he suffered from lymphoma, although his cancer is in recession.

Bob Davis, chairman of the Tennessee Republican party, said the two White House rivals were “good friends and will always remain so, but Fred Thompson is not running against John McCain but for the office of president”.

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