Mr. Thompson, the former Tennessee senator and current presidential question mark, is speaking the next night at the annual dinner of the Lincoln Club of Orange County, an influential conservative group. The scheduling illustrates the political place Mr. Thompson occupies: he is of the presidential campaign, but not in it. Yet.

Making speeches at carefully chosen appearances, doing an occasional interview and fielding questions from Republican congressmen, Mr. Thompson, 64, is running something of a guerrilla exploratory effort. He even weighed in recently on a conservative blog to offer a detailed defense of his ideas on federalism.

Behind the scenes, Mr. Thompson has been consulting with his inner circle — including former Senators Bill Frist and Howard H. Baker Jr. of Tennessee and experienced Washington aides like Mark Corallo, a former Justice Department official — about how he could pull together the money and staff he would need to run.

Those supporters have been reaching out privately to potential contributors to make sure they could still tap into the Republican donor base. Ted Welch, a Nashville businessman and past fund-raiser for Mr. Thompson, has signed on with Mitt Romney and has indicated he intends to stay there. But Mr. Thompson’s supporters said that they thought he could raise the money to be competitive and that they were banking on an Internet effort driven by his star power and dissatisfaction with the current field of candidates.

The informal Thompson team has kicked around names of potential top campaign aides, lawyers and pollsters who might still be available given that so much of the Republican talent pool has already been scooped up. The team has discussed securing office space for a headquarters in Tennessee.

Mr. Thompson’s disclosure that he was treated for lymphoma was seen as more evidence of presidential preparation. And at a private meeting a few weeks ago with House Republicans, he answered questions about his reputation as a man about town during his eight years in the Senate, a period when he was single after his divorce from his first wife. Mr. Thompson was asked bluntly if any activities from his first marriage or his time in the Senate would come back to haunt him or his backers.

According to those attending, Mr. Thompson assured them there were no problems, but conceded that when he was single, “I chased girls and girls chased me.” Mr. Thompson is since remarried, and he and his wife, Jeri, have two young children.

Also watching anxiously are the millions of fans of “Law & Order” on NBC, the show on which Mr. Thompson plays the fictional prosecutor Arthur Branch. The network has not yet announced if it will renew the long-running drama, and it remains uncertain whether equal time issues could spell trouble for the ever-present cable reruns should Mr. Thompson enter the race officially.

His supporters and others who have met with him are convinced that Mr. Thompson is nearing a decision and is likely to become a candidate in the weeks ahead, a probability they see reflected in the higher public profile he has adopted.

“You will see a whole lot of him,” said Representative Zach Wamp, a Tennessee Republican leading an effort to draft Mr. Thompson. “He has the luxury of getting a lot of national attention without being an announced candidate.”

Should he choose to run, Mr. Thompson and his supporters would face the challenge of converting his celebrity profile and Senate experience into a bona fide presidential campaign. Veteran Republican strategists say the logistics of the task should not be underestimated.

“I just wonder how he is going to start this late and have the campaign infrastructure, local organizations on the ground in the early states and raise the money,” said Charlie Black, a senior Republican strategist who is backing Senator John McCain. “Celebrity doesn’t pay for an airline ticket to Des Moines.”

Those encouraging Mr. Thompson say his conservative aura and movie star presence will allow him to catch up quickly. “Fred has a commanding personal presence that makes every other politician jealous,” said Senator Lamar Alexander, Republican of Tennessee. “He attracts people.”

Money and organization are not his only obstacles. Mr. Thompson would have to demonstrate appeal beyond the South, and overcome a reputation lingering from his Senate service that he does not always put his nose to the grindstone.

Mr. Thompson’s allies say that if his chief weakness is that he is considered lackadaisical, he can easily overcome that by mounting a strong campaign. They say he is taking a typical Thompson approach to his future, examining the options from all angles.

“Fred is a big man who moves literally and figuratively at his own pace, he has the appearance of kind of ambling,” said Tom Ingram, a former political adviser to Mr. Thompson who is a chief aide to Mr. Alexander. “It is part of his charm. But people who worked closely with him will tell you he spends hours in his office looking at issues from 360 degrees.”

If Mr. Thompson runs, both supporters and potential rivals say he will be an immediate top-tier candidate, not an unreasonable view considering he is performing strongly in polls even though he is not an announced contender.

Mr. Thompson came into the public eye — and ear, considering his distinctive voice — in the early 1970s when he served as Republican counsel to the Senate Watergate committee. He then took on some lobbying clients, and was later asked to investigate a parole scandal in Tennessee. That episode led to a book and a movie, “Marie,” in which Mr. Thompson played himself, kicking off his acting career.

Elected to the Senate in 1994 to fill the remaining two years of Al Gore’s term after he was elected vice president, Mr. Thompson was seen on Capitol Hill more as an investigator than a legislator.

While in the Senate, he championed campaign finance reform with Mr. McCain, a position that has produced some anxiety among conservatives who otherwise see him as a welcome alternative to Mr. McCain, Rudolph W. Giuliani and Mr. Romney. Since indicating his interest in a presidential bid, Mr. Thompson has also had to push back against challenges to his anti-abortion credentials, but he assured the House members in his session that he was solidly against abortion rights.

Mr. Thompson, who voted to give President Bush authority to invade Iraq in 2002, was running for re-election that year but changed his mind after the loss of a daughter, whose death was later attributed to an accidental overdose of prescription drugs.

The existing campaigns are not eager to criticize Mr. Thompson publicly at this stage. He and Mr. McCain were close friends, and Mr. Thompson had a leadership role in Mr. McCain’s White House campaign in 2000. But his own flirtation with a run against Mr. McCain does not appear to have provoked hard feelings so far.

A top aide for one contender said he thought Mr. Thompson would be stunned by the level of scrutiny he receive in a campaign not only from the news media, but also from the competition. He described Mr. Thompson as the potential Wesley Clark of the 2008 race: a popular figure whose political image and skills have not been tested.