Thompson in '08? His entry would shuffle GOP race.

He has been a lawyer, a US senator, and a TV actor, and now Fred Thompson has made it clear that he's ready to audition for the role of a lifetime, president of the United States.

The 6-foot, 6-inch Tennesseean enters the 2008 race late, but not too late, analysts say, particularly because about half the Republican electorate has indicated to pollsters that the choices so far are less than inspiring. The top tier of GOP candidates all have perceived flaws, and thus Republican activists believe room remains for a straight-ahead fiscal and social conservative who knows how to play to the cameras. The folksy, avuncular manner and Southern twang also don't hurt.

"I think Thompson is going be formidable," says John Geer, a political scientist at Vanderbilt University in Nashville, Tenn.

Former Senator Thompson has not yet formally entered the race, but has explored the possibility for months and is now hiring staff. His advisers have told reporters he will formally announce over the Fourth of July holiday in Nashville. Without spending a penny, he already places in the GOP's top tier of candidates, at times breaking into double digits. The question is how much of that early support is really a vote for "none of the above," rather than an affirmative choice by Republican primary voters.

And, as Gen. Wesley Clark learned four years ago in the race for the Democratic nomination, it's one thing to be wooed by supporters with a draft campaign, but another thing entirely to actually run and open yourself up to the scrutiny and criticism of the news media and opponents. The difference, with Thompson, is that he has political experience and a bit of a shtick, which General Clark did not. Observers expect Thompson to dust off the red pickup truck that made regular appearances during his first Senate race.

Still, he has his work cut out as he seeks to introduce himself to Americans. Even if he's a familiar face to fans of NBC's "Law and Order" as District Attorney Arthur Branch, few voters could get beyond square one on Thompson's issue positions.

"Most people don't know much about Fred Thompson," says Stuart Rothenberg, editor of a nonpartisan political report. "Some know he's an actor, some know he's a senator, [but] I don't think anybody has gone through his political record with a fine-tooth comb. He's hardly an ideologue."

In easy comparisons with the nation's last (and only) actor-president, Ronald Reagan, Thompson probably does not stack up at this stage in that he lacks the defined ideology that turned Reagan the man into Reagan the movement. Thompson, in contrast, garners attention from the figure he cuts, and less from anything he did as a senator. In his eight years in Congress, he was not known as a leader on any particular issues.

"There are plenty of people who find him intriguing or appealing, because they see in him whatever they want to see in him," says Mr. Rothenberg.

Thompson's late entry into the race also raises questions about how much he really wants to be president – and whether he has the fire in the belly to embark on such a grueling race. But, some analysts say, the fact that Thompson was not born wanting to be president, in the manner of a Bill Clinton, does not necessarily hurt him. In Thompson's case, the fact that he has spent much time considering the possibility, being thorough and cautious, could help him.

"Ronald Reagan didn't have the burning desire to be president either," says Mr. Geer. "The American public could find that potentially attractive about Thompson."

Still, Thompson will face a steep task in fundraising, as the top tier of candidates have already raised at least $10 million each. Some analysts note that the Republican field's lower fundraising totals for the first quarter of 2007, in comparison with the Democratic field's take, show that there is a great reserve of untapped GOP money out there for Thompson.

Of the top candidates in the race, former New York City Mayor Rudolph Giuliani enjoys wide name recognition but holds liberal views on social issues, which could kill his chances when voters start tuning in – especially religious conservatives. Sen. John McCain of Arizona, though a conservative on most issues, is also seen as unreliable by the Christian right. Former Massachusetts Gov. Mitt Romney wins points for his executive experience, but his recent shift to the right also leaves some conservatives cold.

Thompson is expected to gain in the polls when (and if) he does formally join the race. And if Thompson remains competitive, showing poll numbers in the top tier, that could deter another lurking Republican, former House speaker Newt Gingrich, from entering the race. Mr. Gingrich has long hinted at launching his candidacy in September, but if Thompson catches on, Gingrich may be frozen out.

"Where is Gingrich going to go?" says Tony Fabrizio, a GOP pollster unaffiliated with any '08 candidate. "He's already 100 percent known by [the party]. It will be tough for him to gain support until other people drop out."

Who Thompson hurts in the existing field and who he helps is open to speculation. Some analysts suggest that Mr. Giuliani gains, because Thompson's entry dilutes the conservative field. But it may be that Thompson takes away from all the front-runners.

Another unknown is how effectively Thompson will use all the modern tools of communication, particularly those that involve cameras. But given his long experience in television and film, Thompson could have a leg up over the other candidates. And for a taste of the Thompson style, the video of the former senator responding to liberal filmmaker Michael Moore about healthcare in Cuba, posted on Youtube.com, is one place to start.

• Zoe Tillman in Washington contributed to this report.