Judge Clarence Thomas has been there before, but it has never mattered as much.

As a young assistant secretary at the Education Department, as head of the Equal Employment Opportunity Commission for 7 1/2 years and as a nominee for the federal appeals court here just 19 months ago, he has testified about 60 times before various congressional panels.

But when his confirmation hearings to become the 106th justice of the Supreme Court open at 10 a.m. today in the Senate Caucus Room, Thomas will undergo a far more intense, wide-ranging and high-stakes inquiry than any the 43-year-old jurist has yet encountered. His chief Senate ally, John C. Danforth (R-Mo.), describes it as a "three-day bar exam on national television."

The expected topics of questioning will range from the specific (Thomas's conduct on various matters at Education and the EEOC, and the newly opened question of his travel reimbursements) to the intellectual (his views, as far as the senators can press him to elucidate them, on various issues of constitutional law).

At times, the proceedings could look more like an advanced jurisprudence seminar than a typical confirmation hearing, as Thomas is questioned about the "natural law" method of constitutional interpretation he has set out in speeches and law review articles. Committee Chairman Joseph R. Biden Jr. (D-Del.) is particularly interested in the topic and is expected to quiz Thomas extensively about it.

As was the case with his predecessor before the committee, Justice David H. Souter, the confirmation, in the analysis of strategists for both sides, is largely Thomas's to lose.

As part of his summer-long preparation, Thomas watched videotapes of Souter's committee appearance last September, in addition to the usual drill of three or four mock hearings -- what Danforth describes as "batting practice."

Comparing the situation of the two nominees, White House congressional liaison Fred McClure said, "With Justice Souter I had somebody who had spent a great deal of his life dealing with the law but who had not spent a great deal of time under the glare and lights of a confirmation hearing.

"With Judge Thomas I have somebody who's gone through four confirmation hearings . . . but also someone who has a lot more of a public, written track record, more of a plethora of documents which could possibly be the subject of a line of questioning."

The goal of Thomas's opponents is to employ that paper trail to build a case that Thomas is a dangerous ideologue with a history of disregarding laws with which he disagrees.

"We expect Clarence Thomas to do a good job and be a good witness," Ralph G. Neas, executive director of the Leadership Conference on Civil Rights, said last week. But, he said, "At the end of that process it's going to be very clear that Clarence Thomas is a right-wing conservative."

Danforth said yesterday that Thomas's opponents have been unable to paint the nominee as someone who is "on an ideological toot." Danforth and others will seek to portray Thomas as a centrist who would bring a needed perspective to the court, the product of the segregated South replacing Justice Thurgood Marshall as only the second black to serve on the high court.

President Bush, who called Biden yesterday to lobby on Thomas's behalf, sounded that theme in a radio address Saturday to promote the nomination. "When you hear or see coverage of those hearings, think of your sons, your daughters, your loved ones and their voyage into a tough world," he said. "Then think of this extraordinary man, who conquered deprivation without self-pity or complaint. And think of what it means to appoint to our highest court a man who appreciates the real glories of our form of government and understands the real difficulties our nation faces."

To combat what Neas has called the White House "fluff campaign," critics face an uphill battle.

No key senators have announced opposition to Thomas, and the national attention that was already focused on nominee Robert H. Bork by the time of his hearings four years ago has not yet materialized in the Thomas nomination.

"By the time Bork testified, the burden was on Bork," said Mark H. Gitenstein, former chief counsel of the Senate Judiciary Committee and author of a forthcoming book on the Bork battle. "This time around the burden is not yet on Thomas to defend himself."

Part of opponents' effort to shift that burden will be made by focusing on Thomas's record at Education and the EEOC. But a potentially critical component is Thomas's larger constitutional philosophy, particularly his assertion in various speeches and articles that the Constitution should be interpreted in light of a "natural" or "higher law."

Natural law is a fuzzy and malleable concept that has been endorsed by liberals as well as conservatives. In contrast to the strict constructionist view that the Constitution must be interpreted by sticking closely to its text, natural law holds that the Constitution also protects rights that are not specifically "enumerated," or set out, in the words of the document.

Thomas's version of natural law closely resembles that set out by one of its most prominent conservative adherents, political theorist Harry V. Jaffa of Claremont College in California. Indeed, Thomas did not discuss natural law in his speeches or writings until 1986 and 1987, after he hired two former Jaffa students, Ken Masugi and John Marini, to be his special assistants.

Thomas referred to Jaffa in a November 1987 speech at Claremont as one of the "defenders of freedom" who had helped refine the lessons of "what my grandparents and my early teachers imparted."

Much of Thomas's discussion of natural law repeats Jaffa's views that the Constitution must be interpreted in light of the Declaration of Independence and its reference to the inalienable rights of life, liberty and the pursuit of happiness.

Thomas appears to have been drawn to Jaffa's discussion of natural rights as a source for reading the original Constitution to condemn slavery.

Jaffa also has expressed other, more controversial views, which Thomas has not commented on to date, but which he could be called on to embrace or repudiate in the confirmation hearings.

Jaffa has stated strongly that homosexuality, just like slavery, is against the laws of nature. Of the AIDS epidemic, he said, "It would certainly seem that nature had an interest in the morality that is conducive to the family, and punishes behavior inimical to it."

He has criticized the Supreme Court's 1987 decision striking down a Louisiana law requiring the teaching of creationism, writing, "How can we exclude creation from our explanation of the world while declaring that 'All men are created equal,' and are 'endowed by their Creator with certain inalienable rights'?"

And he has appeared sympathetic to the notion that the Declaration's protection for the right to life would extend to fetuses, a view explicitly set out in an article by conservative businessman Lewis Lehrman that Thomas praised in a 1987 speech.

Writing about how the death penalty could be reconciled with the Declaration's right to life, Jaffa said: "The right to life which we are bound to respect is, as such, a right of innocent life. The life of a murderer -- or tyrant -- is not on a level with that -- for example -- of an unborn child."

On the other hand, Jaffa has indicated that the right to privacy announced by the Supreme Court in Griswold v. Connecticut, striking down a Connecticut law making it a crime for married couples and others to use contraception, could be justified on a natural law theory.

Former Justice Department official Charles J. Cooper -- a vehement opponent of the natural rights school of thought although he is a staunch Thomas supporter -- recalled a series of brown-bag lunches in the years after Masugi and Marini came to the EEOC in which the two officials and their aides would debate natural rights.

Since then, Cooper said, Thomas "has retreated substantially in his own thinking from the full-bodied Harry Jaffa position that at the time we were arguing about." Cooper said Thomas was simply exploring Jaffa's philosophy and "developing his own thoughts" and has "evolved" in the last few years to more of a traditional conservative position.

Thomas himself is likely to seek to play down his natural law views -- and to ally himself with, among others, Justice Marshall, the man he has been tapped to replace, in embracing a natural rights philosophy.

"Let me tell you about natural law," Thomas told Legal Times in a photo session last Wednesday, noting that Marshall invoked natural law arguments against segregation in his brief to the Supreme Court in Brown v. Board of Education.

"Martin Luther King talked about natural law," Thomas added. "The founders of our country, the drafters of our Constitution, they all were influenced by it."

Thomas suggested that liberal opponents were simply using natural law as a peg to get him. "I'd still be catching it," he complained, "if I'd embraced nihilism instead of natural law."

Staff writer Helen Dewar contributed to this report.