[##_1L|1216845435.jpg|width="110" height="133" alt=""|_##]Thompson's entry will have an immediate impact on the battle for the GOP nomination, adding a fourth candidate to the field's top tier, which includes former New York mayor Rudolph W. Giuliani, Sen. John McCain of Arizona and former Massachusetts governor Mitt Romney. All three have struggled to win the confidence of conservative Republicans. Thompson will attempt to make the case that he is the true heir to the mantle of Ronald Reagan and, if successful, would become a formidable candidate for the nomination. But Republican strategists cautioned that Thompson will need a more refined message and an error-free start to live up to the publicity surrounding his all-but-certain candidacy.

"That's what the campaign will be all about for him -- persuading a significant portion of the party that he truly is the right leader for a set of issues and an outlook on the world," said Whit Ayres, a Republican pollster.

By tomorrow, aides said, the actor and former senator from Tennessee will incorporate a committee called Friends of Fred Thompson and will begin actively raising money for a White House bid. He launched the fundraising effort this week in a conference call with more than 100 supporters, whom he has dubbed his "First Day Founders."

Within the next few weeks, advisers say, a real campaign will take shape, even without a final decision or formal announcement. A Web site will be posted, campaign headquarters will be selected, and a staff will be hired. The signature red pickup truck from Thompson's Senate campaigns will be dusted off.

A senior adviser, who spoke on the condition of anonymity because Thompson has not formally announced his intentions, said he is confident about the future.

"This is not someone who is awkward in his own skin," the adviser said. "This will not be a D.C.-centric campaign. He has natural assets that appeal to conservatives, but at the same time he is not threatening to independents and Democrats."

Thompson will give a speech in Virginia this weekend and is scheduled to appear next month on "The Tonight Show With Jay Leno." An announcement could come as soon as the first week of July, using the hoopla of the national holiday as a backdrop. But those plans are in flux and could change, two sources said yesterday. One source said a formal announcement is likely to come "around that time."

As a lawmaker, Thompson exuded a folksy charm that supporters say could help him capture the attention of many Republican primary voters. His decades of movie and television appearances give him an immediate national presence that rivals that of the others in the campaign. Thompson has played District Attorney Arthur Branch on "Law & Order," but he told the television show he will not return in September, although he did not indicate any political intentions, producer Dick Wolf said in a statement.

Thompson, a senator from 1994 to 2003 and a guest host on Paul Harvey's show on ABC Radio, has already begun to reach out to party conservatives. He has been outspoken in his support of the war in Iraq and blasted the immigration deal reached in the Senate. He recently used a spat with liberal filmmaker Michael Moore to draw attention on conservative blogs, issued a Web video featuring himself chomping a cigar and chiding Moore for going to Cuba to film part of his new documentary.

Republican strategists predicted yesterday that Thompson will get an immediate boost in the polls by entering the race. "I think overnight he becomes the alternative," one strategist said.