WASHINGTON — At the pinnacle of Fred Thompson’s career in the Senate, a conservative activist was so disappointed in the senator that the activist put theTennessee Republican on a “wanted” poster. Senator Lott of Mississippi, the GOP leader at the time, had been fuming at him. Republican colleagues were steamed when Mr. Thompson threw his weight behind a campaign finance bill that conservatives loathed.

“Has Fred Thompson Blown It?” blared a headline in the Weekly Standard magazine, accusing him of squandering an opportunity to use a set of 1997 hearings to nail Democrats for illegal fund raising.

A decade later, as Mr. Thompson prepares to formally announce his bid for the 2008 presidential nomination, he is being promoted as a godsend for conservatives dissatisfied with the established field of Republican candidates.

But during his eight-year Senate career, his only stint in elected office, Mr. Thompson was far from a champion of the party’s conservative core. In the two enterprises where he made his biggest mark — the fund-raising hearings of 1997 and the successful drive for campaign finance overhaul — Mr. Thompson infuriated conservatives.

While he compiled a largely conservative voting record, he also carved out a maverick profile akin to that of Senator McCain, a Republican of Arizona, with whom he co-sponsored a landmark campaign finance measure along with Senator Feingold, a Democrat of Wisconsin.

In the 2008 campaign, conservatives are looking to Mr. Thompson as an alternative to Mr. McCain and other GOP candidates whom they consider unreliable allies on key issues.

An actor, lawyer, and lobbyist, Mr. Thompson seems to have earned more forgiveness than Mr. McCain for breaking with conservative dogma, in part because his maverick streak was tempered by an easygoing manner and a willingness to stick with the GOP on most issues. But it may also be because conservatives who back him now know less about Mr. Thompson’s Senate record than they do about his performance as a district attorney in the television hit “Law & Order.”

“He carries the same baggage that McCain carries,” said James Bopp Jr., an anti-abortion activist who is backing a former Massachusetts governor, Mitt Romney, for the GOP nomination. “Time does dim memories, and people need to be reminded of his support for Mr. McCain-Feingold.”

“Thompson had a chance to show leadership and did not,” said Larry Klayman, the conservative lawyer who issued the “wanted” poster to criticize Mr. Thompson for not running more aggressive hearings on President Clinton’s fund-raising.

“I would not vote for him for president.”

That statement underscores one of the biggest questions confronting Mr. Thompson: Will conservatives continue to be attracted to him once they know more about his record? Campaign finance was not the only issue that put Mr. Thompson at odds with conservatives during his Senate years.

When the Senate voted in 1998 on impeaching Mr. Clinton on charges arising from his affair withanintern, Mr. Thompson was one of 10 Republicans who voted against conviction on one of the two counts. And Mr. Thompson, a former trial lawyer, opposed elements of a GOP effort to curb lawsuits. Also, though he voted with conservatives on many social issues, he did not put those issues front and center.

Abortion may prove to be an unexpectedly touchy area. He built a consistent anti-abortion voting record in the Senate, but he also opposed a constitutional amendment to ban abortion.

And questions about his commitment to the anti-abortion cause have arisen from claims by a family-planning group and others, reported in the Los Angeles Times, that Mr. Thompson took a paid assignment in 1991 to lobby the administration of President George H.W. Bush to loosen an abortion restriction.

Lately, Mr. Thompson has been backpedaling on his support for the Mr. McCain-Feingold measure — which sought to limit the influence of big campaign donors in politics — saying that some parts of the law were not working as he hoped.

But Mr. Thompson was a central architect, not a casual supporter, of the measure. Republican leaders and conservative activist groups bitterly opposed the measure, which they believed would disproportionately hurt the GOP and its allies.

Mr. Thompson’s focus on government overhaul was a logical outgrowth of his first run for the Senate, in 1994. Although he had spent years as a lobbyist and was a senior Senate aide in the 1970s, Mr. Thompson ran as a reform-oriented outsider railing against the Washington establishment. He was not seen as a hard-line conservative but as more moderate in style and politics, in the mold of his Tennessee mentor, Senator Howard Baker Jr.

Even before he was sworn in, a former aide said, Mr. Thompson decided to sign on to Mr. McCain-Feingold. Mr. McCain had a desk next to Mr. Thompson’s on the Senate floor and became a good friend. When the campaign finance bill was formally introduced in late 1995, Mr. Thompson was one of the few original Republican co-sponsors.

When the measure was on the Senate floor in 2001, Mr. Thompson was part of a core group of about 10 senators that met every morning to strategize before the day’s debate. He was so wedded to the issue that he sometimes complained that his name was not included in its moniker, according to a Senate aide who worked with him on the legislation. And when the law was challenged before the Supreme Court, he filed a friend-of-the-court brief supporting it. Mr. Thompson also entered the money-in-politics debate in 1997, when he oversaw hearings by the Senate Governmental Affairs Committee into alleged fundraising abuses by Democrats during the 1996 campaign. When Lott picked him to head the investigation, Mr. Thompson seemed like the perfect choice: He brought the star power of his acting career and the gravitas of his experience on the staff of the Watergate committee.

Republicans hoped the hearings would hit political pay dirt — a scandal “bigger than Watergate,” as then-House Speaker Newt Gingrich, a Republican of Georgia, put it — by probing claims of access-peddling by the Clinton White House and efforts to funnel foreign cash into Democratic campaigns.

The way Mr. Thompson conducted the hearings may raise questions about whether he has the zest for cut-and-thrust partisanship that many conservatives want in their leaders: Although conservatives wanted to keep the focus on Mr. Clinton and the Democrats, Mr. Thompson defied Mr. Lott and broadened the scope of the investigation, giving Democrats opportunities to question GOP practices.

Among the witnesses called was a former Republican Party chairman, Haley Barbour, who had to answer questions about accepting a foreign-backed loan to a GOP policy arm. Mr. Thompson allowed Democrats to subpoena conservative groups such as the Christian Coalition and the National Right to Life Committee. The groups’ leaders were furious.

“It was a highly objectionable investigation,” said Mr. Bopp, who represented those groups and refused to comply with the subpoenas, without any consequence from the committee. “It was a fishing expedition, highly intrusive and unconstitutional.”