In a conference call on Tuesday, 75 to 100 Republicans who had offered to back the politician better known for his acting career were urged to raise roughly $50,000 each for the financial underpinnings of a Thompson bid that has been in the making for months. Five people closely involved in the emerging campaign discussed the plans in interviews Wednesday but asked not to be identified because they were not authorized to talk publicly about the details.  Participants on the call said Mr. Thompson was enthusiastic about becoming an alternative to current contenders led by former Mayor Rudolph W. Giuliani of New York, Senator John McCain of Arizona and former Gov. Mitt Romney of Massachusetts. They said he was not ready to formally announce without evidence that he could draw the necessary financial support.  “You have to raise money to be competitive even if you are Fred Thompson,” said Representative Zach Wamp, a Tennessee Republican who has led an effort to draft Mr. Thompson.  In another development on Wednesday, Mr. Thompson asked to be released from his role on “Law & Order” on NBC, The Associated Press reported. The show’s executive producer, Dick Wolf, said Mr. Thompson had told him that he “has not made a firm decision about his political future” but that he was concerned about “the creative and scheduling constraints of the upcoming season.”  By joining the race late, Mr. Thompson may face questions about his ability to compete against the top-tier Republican candidates who have already raised at least $10 million each. He will also not participate in the Republican debate in New Hampshire on Tuesday.  Tony Fabrizio, a Republican pollster and strategist who is not working for any candidate, said he was “surprised and a little sad” that Mr. Thompson had not declared himself a full-fledged candidate. Mr. Thompson has faced questions about his political commitment stemming from his days in the Senate.  “Thompson will be competing against three guys who have been running flat out for months and working their tails off for well over a year,” Mr. Fabrizio said. “If you’re not going to get in this race and double-time on work, effort and commitment, how do you expect to win? You need to send signals that say, ‘I’m here and I’m going to win.’ “  Although Mr. Thompson planned in the coming days to create what is known as a “testing the waters” committee that would allow him to raise and spend money, members of Mr. Thompson’s inner circle said that if the next few weeks were encouraging, he could officially enter the race as early as July.  “Fred Thompson is doing everything he has to do to gauge support to come to a final decision in a timely manner,” said Mark Corallo, who has been acting as his spokesman.  The intense interest in Mr. Thompson’s trajectory reflected concern in the rival Republican camps about his possible appeal across the South, among conservative and evangelical voters, who might be attracted to his mainstream conservative stances, like opposition to abortion and strong support for states’ rights.  “There are still a lot of undecided voters in South Carolina, and Fred Thompson comes with enormous appeal — I’d call it Ronald Reagan-like appeal,” said Katon Dawson, the chairman of the Republican Party in South Carolina, which holds one of the first presidential primaries. Allies of Mr. Thompson said they were confident he would attract big-name contributors and present and former office holders now supporting other candidates. But potential competitors questioned his ability to make such inroads.  Kevin Madden, a spokesman for Mr. Romney, disputed that Mr. Thompson might peel away conservative and evangelical voters whom the former Massachusetts governor has been courting. Mr. Madden noted that Mr. Romney might make his own gains in Thompson country when he appears Saturday as the keynote speaker at the Tennessee Republican Statesmen’s Dinner in Nashville.  “The governor looks at these events as opportunities — it could be a Romney crowd by the end of the night,” Mr. Madden said.  Anthony Carbonetti, an adviser to Mr. Giuliani, called Mr. Thompson “a good senator who was entitled to run.” But Mr. Carbonetti played down any concern in the Giuliani campaign that Mr. Thompson would run strongly among voters who are deeply concerned about national security and want a candidate who is socially conservative as well. Mr. Giuliani supports abortion rights.  Mr. Thompson, who was Republican counsel to the Watergate committee, served in the Senate for eight years before retiring in 2002 and was an ally of Mr. McCain’s. Mr. Thompson’s support for the campaign finance measure authored by his colleague from Arizona has brought criticism from conservatives, who have also raised some questions about his anti-abortion credentials.  For the past few months, Mr. Thompson has been running something of a shadow campaign, courting potential contributors, giving selected interviews and speeches, weighing in on conservative blogs and lining up staff members. At the same time, he has continued on the television show and as a commentator on ABC radio as a replacement for Paul Harvey. In April, he announced that he was in remission from lymphoma, a cancer he has had for more than two years.  The testing-the-waters group, to be formed as Friends of Fred Thompson Inc., allows him to take his political activities to another level but does not commit him entirely. It also frees him to gauge his support while leaving time for him to sort out his lucrative acting and commentating careers, which would have to be curtailed if he became a candidate or create equal-time issues.  The committee being set up does not yet fall under the Federal Election Commission reporting requirements, but if he enters the race all financial activity would have to be reported by mid-October.  With that in mind, Thompson advisers said the budding campaign would follow conventional campaign rules, including a $2,300 limit per individual contribution for the primary. Allies said that on the conference call, backers were urged to collect at least $46,000, or maximum donations from 20 individuals each. Advisers said several already had larger commitments.  “Money is a good test of whether the interest is as real as it seems,” one Thompson adviser said.  A version of this article appears in print on , on Page A1 of the New York edition with the headline: Stir in G.O.P. As Ex-Senator Moves to Run. Order Reprints| Today's Paper|Subscribe
Fred D. Thompson could shake up a field of candidates that has failed to strike a chord with the Republican base.