Fred D. Thompson had one central strategic goal as he formally began his presidential campaign on Thursday: to win over conservatives who are disheartened at their current choice of Republican candidates by positioning himself as the ideological and stylistic heir of Ronald Reagan.

Mr. Thompson is certainly conservative, and has been throughout his public life — particularly on the question of federalism, the size of government, tax cuts and his unwavering support for the war in Iraq.

Biographically and stylistically, Mr. Thompson, another former actor trying to become president, recalls the easygoing manner that Reagan used to advocate conservative solutions to the nation’s challenges, as he made clear with his announcement speech in Des Moines on Thursday. He spoke of “common-sense conservative beliefs,” including the notion that “we still get our basic rights from God, not government.”

Yet in some notable ways, Mr. Thompson is different from Reagan, and he has at times deviated from the orthodox conservatism that Reagan, after his death and nearly two decades removed from his presidency, has come to represent.

Mr. Thompson, the former senator from Tennessee, has at times voted in support of affirmative action, at other times against it; Reagan’s Justice Department consistently championed efforts to eliminate it. Mr. Thompson, a former trial lawyer, has voted against efforts to impose federal caps on punitive damages and lawyers’ fees, a central part of the conservative agenda.

Although he consistently voted in favor of restrictions on abor-tion during his eight years in the Senate, his position has not always been as clear-cut, suggesting that he evolved on the issue much the way Reagan did. In questionnaires Mr. Thompson answered when first running for the Senate in 1994, he checked a box saying he believed abortion should be legal in the first three months of pregnancy.

Along with Senator John McCain of Arizona, Mr. Thompson was a sponsor of the campaign finance bill that was strongly opposed by conservatives and remains a target of their ire. He split his votes in the Senate on the articles of impeachment against President Bill Clinton.

“Thompson was never an ideological conservative,” said Lou Cannon, a biographer of Ronald Reagan who has also written about Mr. Thompson. “He’s a very mainstream Republican. Reagan was more conservative.”

Mr. Thompson’s announcement, in contrast to the announcement speeches by some of his rivals, made no mention of Reagan as he declared his candidacy on his Web site and in Des Moines on Thursday. One of his aides said there was no reason to force the comparison, since Mr. Thompson’s career and political positioning would lead analysts and journalists to make it on their own.

But just in case anyone might miss the point, the campaign introduced Mr. Thompson at a rally in Iowa on Thursday with a video that included a photo of him with Reagan.

Conservatives have yet to coalesce around any one of the Republican candidates. Rudolph W. Giuliani, the former mayor of New York, supports gay rights and abortion rights; Mitt Romney of Massachusetts has shifted his position on abortion over the past five years; and Mr. McCain is viewed with suspicion by conservatives for, among other reasons, his advocacy of campaign finance restrictions and his stance on immigration.

The qualms about the ideological credentials of the other candidates have given Mr. Thompson an opening. Since leaving the Senate, he has continued to burnish his conservative credentials by speaking out on issues of concern to the Republican base, like immigration, taxes and terrorism, through a Web column and a syndicated radio commentary.

Some Republicans said that electability in the general election, rather than adherence to a conservative line, might prove to be the most powerful factor in the selection of a nominee. And conservatives who spoke optimistically about Mr. Thompson were as likely to talk about his presentation, and compare it to that of Reagan, as they were to talk of his politics.

“It’s a fair comparison because Fred Thompson’s ability to communicate — his likability — just comes across, and that is what Ronald Reagan had,” said David N. Bossie, president of Citizens United, a conservative group.

Martin Anderson, a longtime associate and a biographer of Reagan, said he liked Mr. Thompson but was baffled by the comparisons to Reagan. When Reagan ran for the presidency, Mr. Anderson said, he laid out a clear agenda of what he intended to do, starting with systematic reductions in taxes.

“I don’t know where Thompson is on any of these things; I don’t know what he is going to do,” Mr. Anderson said. “Everyone says they are like Reagan. So far, the only way I’ve seen that Thompson is like Reagan is that he did some acting.”

Still, Mr. Thompson was generally viewed as a center-right conservative over eight years in the Senate. He repeatedly described himself as conservative in his remarks on Thursday, and aides said his appeal to that segment of the party electorate was critical to the success of his late-starting campaign.

Since he began exploring his candidacy, Mr. Thompson has criticized Republicans and Democrats alike for failing to rein in spending. Had he been in Washington, he said, he would have opposed the White House’s successful push to add a Medicare drug benefit. He has embraced conservative positions on immigration and gun control measures.

More than anything, he points to his long-standing conservative view of federalism as informing his views on issues like abortion and gay rights. The philosophical stance has often led him to stake out lonely positions: he voted against setting a 0.08 blood alcohol level as the national drunken-driving standard, as well as making it a federal crime to carry a gun in or a near a school.

“Thompson, like Reagan, has an acute understanding of federalism,” said Craig Shirley, a conservative consultant and the author of a book on Reagan. “I think the tell-tale vote for me and a lot of conservatives was when he was in the Senate and there was that vote on the D.U.I. limit.”

Still, Mr. Shirley said, it is a mistake to draw too much of a comparison between Reagan and Mr. Thompson, saying that the two men had vastly different relations with the conservative movement. That could prove significant as Mr. Thompson moves to win conservative support against a concerted appeal for the same voters from Mr. McCain, Mr. Romney and Mike Huckabee of Arkansas.

“Long before Reagan won the nomination, he had a 30-year investment in the conservative movement, and Thompson does not,” Mr. Shirley said. “Reagan had so much invested in the movement, and the movement had a lot invested in him. That’s one big difference.”