WASHINGTON – Larry Thompson appears on many lists as a potential appointee to a future vacancy on the Supreme Court, where he would be the second black justice, but he remains in many ways a man of mystery.

As a former deputy attorney general, Mr. Thompson served as second-in-command to John Ashcroft, and he is a friend of the conservative Justice Thomas. He is a Republican not known for wearing an ideology on his sleeve, or expressing personal opinions on sensitive subjects to his colleagues. A senior executive at PepsiCo since August, Mr. Thompson, 59, has never served as a judge. Nor has he produced a paper trail of scholarly articles or judicial opinions by which his potential supporters or critics could judge him.

Should he be nominated, court watchers across the political spectrum say they would have many questions about his legal philosophy, political leanings, and his role in the administration’s terrorism policies.

President Bush set tongues wagging in April when he invited Mr. Thompson to join him at an event in Buffalo to celebrate the anniversary of the Patriot Act. “Larry, we miss you over there,” Mr. Bush said at the ceremony. “Don’t get too comfortable.”

The comments sparked speculation that he would be a candidate for attorney general. When Alberto Gonzales was nominated instead, speculation turned to the top court.

“He would be a phenomenal nominee,” said Jay Sekulow, the chief counsel for the conservative law firm the American Center for Law and Justice, who has known Mr. Thompson for 20 years. “He’s a lawyer’s lawyer – the kind of lawyer you go to if you are a lawyer and you need advice,” Mr. Sekulow said.

It is a common refrain among people who know him.

“If the chips were down, I would want him to be my counsel,” said David Shipley, the former dean of the University of Georgia Law School, where Mr. Thompson has taught.

Few question the breadth and solidity of his legal experience, which has included successful tenures as a federal prosecutor in Atlanta who went after fraud, corruption, and terrorist acts by members of the Ku Klux Klan.

The son of a railroad laborer and a part-time cook from Hannibal, Mo., Mr. Thompson attended segregated schools as a child. He graduated from Missouri’s Culver-Stockton College, earned a master’s degree from Michigan State and a law degree from the University of Michigan.

He spent 16 years as a white-collar defense attorney and taught law before joining the Justice Department. There he worked on anti-terrorism policy and headed a task force that cracked down on corporate crime in the wake of the Enron scandal – coordinating investigations into Enron, WorldCom, and Martha Stewart, among others. He also worked to stiffen federal penalties and encouraged jail time for white-collar criminals.

During his smooth confirmation hearing for that job in 2001, Mr. Thompson was praised for his “obvious brilliance” by a frequent critic of President Bush’s appointees, Senator Leahy, a Democrat of Vermont.

Senator Miller, a conservative Democrat from Georgia, said at the time, “He comes with no agenda.”

He was confirmed by a unanimous Senate.

“No one could say he is a zealot or that he doesn’t have a distinguished background,” said the director of the Center for Legal and Judicial Studies at the conservative Heritage Foundation, Todd Gaziano.

Mr. Thompson, who has spoken out against racial profiling, was endorsed at the time by both the Fraternal Order of Police and the National Association of Criminal Defense Lawyers.

Neither liberal nor conservative court-watching groups have compiled extensive dossiers on him.

“If he worked as John Ashcroft’s deputy, I assume he is a solid conservative. I can’t say much more without getting into speculation,” said the executive director of the Committee for Justice, a group that advocates for the president’s judicial appointees, Sean Rushton.

“He has been deputy attorney general in the Bush administration, and his actions there would come under great scrutiny,” said Eliot Mincberg, the general counsel to the People for the American Way, a liberal group that researches nominees. But so far he is a potential nominee “about which little is known,” Mr. Mincberg said.

Liberal groups say they are concerned that he is a member of the Federalist Society, and that he has sat on the board of the Southeastern Legal Foundation, a conservative litigation group.

They also point to Mr. Thompson’s role as a witness and active adviser for Justice Thomas during his heated confirmation hearings.

But Mr. Thompson’s views on issues such as abortion, affirmative action, or constitutional interpretation are not well known, even to his fans.

“He is not an ideologue, or a person who seems zealous to accomplish some end,” said a professor of law at the University of Georgia, C. Ronald Ellington. “He has a deep respect for the rule of law and for the courts and the legislative process, and our form of government,” he said.

Mr. Thompson’s lack of judicial experience is not seen as an obstacle to a Supreme Court appointment. Earl Warren was made chief justice without it, several legal scholars pointed out.

“The kind of responsibility he had as deputy attorney general really familiarized him with federal law issues that come before the court,” Mr. Gaziano of the Heritage Foundation said.

Mr. Thompson shared his 2001 Senate confirmation hearing with then nominee for solicitor general, Theodore Olson. Questions for the more partisan Mr. Olson dominated the debate, while Mr. Thompson was widely praised.

“If I were the president, I wouldn’t hesitate about Larry Thompson. I don’t know what the baggage would be. I just don’t think there would be much – unless some people said guilt by association with things that transpired with Ashcroft as attorney general,” Mr. Shipley said.

In public speeches, Mr. Thompson has made clear that he agreed with the Justice Department’s change in philosophy for confronting the threat of terrorism, shifting the department’s emphasis from prosecution to preemption and investigation of potential terrorists.

At a confirmation hearing, he would likely be asked to explain what role, if any, he played in controversial decisions involving detainees.

“If he were nominated to the Supreme Court, we would want to know what his involvement was in the decision to send individuals from U.S. custody to the custody of other governments where they have been mistreated,” said the Washington director of the group Human Rights First, Elisa Massimino.

Mr. Thompson has publicly criticized recent Supreme Court decisions granting greater rights to war-on-terror detainees than the Bush administration had recognized as having “completely failed” to give the executive branch adequate direction about the actions it can take to prevent future terrorist attacks.

“I don’t think the courts have the expertise to conduct war,” he said in a speech.