President Bush yesterday chose Clarence Thomas, a conservative black federal appeals court judge, to replace Thurgood Marshall on the Supreme Court, saying he is "the best person at the right time."

Thomas, chairman of the Equal Employment Opportunity Commission under President Ronald Reagan, has for years challenged civil rights leaders over workplace preferences for minorities or women and school busing for desegregation. A 43-year-old Roman Catholic, he would be one of the youngest justices ever to join the Supreme Court.

Conservatives were delighted with the nomination of a longtime favorite, while some Democrats expressed fears that Thomas, whose views on abortion are not publicly known, would help overturn Roe v. Wade, the 1973 decision that established a constitutional right to abortion.

Both sides acknowledged the power of his personal history. He was raised in poverty in Savannah, Ga., by a nearly illiterate grandfather, who he said stressed "God . . . school, discipline, hard work and 'right from wrong.' "

"Judge Thomas's life is a model for all Americans, and he's earned the right to sit on this nation's highest court," said Bush, who insisted race played no part in his choice of a black judge to replace Marshall, the first black justice on the court.

Senate Minority Leader Robert J. Dole (R-Kan.) praised Thomas as "a man whose very life exemplifies the American dream."

Although opposition to his nomination seemed muted yesterday, some senators and civil rights groups predicted that Thomas will face tougher confirmation hearings than did David H. Souter, Bush's first appointment to the Supreme Court. "I'm through reading tea leaves and voting in the dark. . . . I will not support yet another Reagan-Bush Supreme Court nominee who remains silent on a woman's right to choose {an abortion} and then ascends to the court to weaken that right," said Sen. Howard M. Metzenbaum (D-Ohio), who cast the lone vote against Thomas's nomination as an appellate judge.

Thomas avoided giving his potential opponents any ammunition yesterday as he and Bush addressed reporters in front of the wood-shingled cottage that serves as Bush's office at his family's oceanfront home in Kennebunkport, Maine. Thomas restricted his comments to the pivotal role his grandparents, his mother and the Catholic nuns played in his rise.

"As a child I could not dare dream that I would ever see the Supreme Court, not to mention be nominated to it," Thomas somberly said. "Only in America could this have been possible."

His voice choked with emotion as he acknowledged his grandparents, and he paused for several seconds, unable to continue, while Bush looked into the distance.

Administration officials said Thomas, who Bush appointed to the D.C. Circuit Court of Appeals only 15 months ago, emerged as the front-runner almost as soon as Marshall announced his retirement Thursday. The process of selecting him was characteristic of the Bush administration: it involved only a tiny circle of aides and was marked by tight secrecy.

The other finalists were Texas appellate judges Edith H. Jones and Emilio M. Garza, both from the 5th Circuit Court of Appeals, administration officials said. Bush aides argued that Thomas was less controversial than Jones, and more experienced than Garza, sources said.

Administration officials said Bush concentrated almost exclusively on minority or female candidates. Bush, however, told reporters, "The fact that he is black and a minority has nothing to do with this sense that he is the best qualified at this time. I kept my word to the American people and to the Senate by picking the best man for the job on the merits. And the fact that he's a minority, so much the better."

Democrats acknowledged yesterday that the fact that Thomas is black will make it difficult for civil rights groups to make a compelling issue of his opposition to many forms of affirmative action.

"Anyone who takes him on on the subject of civil rights is taking on the grandson of a sharecropper . . . because that person wants quotas and preferential treatment," said Sen. Orrin G. Hatch (R-Utah.).

It may also be difficult for Democrats to carry through on their promises to vigorously question Bush's choice because "the question will be, is a higher standard being applied to this guy than was applied to David Souter, and why," said one Senate Democratic aide. "If Souter was confirmable with non-answers, why isn't Thomas?"

Neither Senate Majority Leader George J. Mitchell (D-Maine) nor Senate Judiciary Committee Chairman Joseph R. Biden Jr. (D-Del.) gave clues to their reaction yesterday, saying only that the nomination would be promptly considered.

Thomas has described himself as a firm advocate of a "colorblind society." "Racial quotas and other race-conscious legal devices only further and deepen the original problem," he wrote in 1987.

"Today . . . color conscious means something favorable to us. We have set-asides, we have affirmative action. . . . I firmly believe that just as we can use it for us it's going to be used against us again," he told an interviewer in 1983.

Thomas has often said that no government program can replace the kind of self-discipline instilled in him by his grandfather, who taught him: "You had to get up, had to go to work." Only his grandfather's philosophy saved him from a life like that of his sister, who was raised by other relatives and now supports four children on welfare, he said in 1983.

At various points, Thomas has questioned rent control, minimum wage laws and enterprise zones to redevelop slums.

As chairman of the EEOC from 1982 to 1990, Thomas drew fire for what critics called a "dismal" failure to enforce anti-discrimination laws. Civil rights groups charged he let thousands of age-discrimination complaints lapse for lack of action. Thomas blamed the growing backlog of unaddressed cases during his tenure on a lack of funds.

Thomas's critics did not sway Senate Judiciary Committee members in February 1990, when they voted 12 to 1 to approve his nomination to the D.C. Circuit. But several senators, including Biden, warned Thomas that they would scrutinize him far more carefully if he came back to them as a Supreme Court nominee.

His most likely opponents at the confirmation hearings, expected to take place in September, are activists for abortion rights and organizations for the elderly.

Kate Michelman, executive director of the National Abortion Rights League, said in a statement: "Never again should senators confirm a nominee to the U.S. Supreme Court who has no record and provides no answers about his commitment to equal justice and fundamental rights."

During his 15 months on the bench, Thomas has written 18 opinions on issues ranging from the Interstate Commerce Commission's jurisdiction over a passenger ferry to a complaint that a criminal defendant's rights against self-incrimination were violated at his trial on cocaine distribution charges.

None of the opinions, said Bruce Fein, a conservative constitutional expert, "were of great moment. He's going to be almost as tabula rasa as David Souter."

Thomas's nomination was set in motion only a few hours after Marshall's resignation, when Attorney General Dick Thornburgh interviewed him. Some administration officials said they believed that Garza would be a better political choice, because any opposition from the Democrats would seem to further link the party's civil rights policy to the interests of blacks.

Garza's appointment also would have been a nod toward a voting bloc that the administration has been courting vigorously. One administration official said Thomas won out over Garza because of a "semiconscious sense . . . this was a black man to be replaced," then immediately backpedaled, saying: "Strike that. He was the best person."

Bush settled on Thomas about 3 p.m. Saturday, during a conference call with White House Counsel C. Boyden Gray, White House Chief of Staff John H. Sununu and Thornburgh.

Bush telephoned Thomas on Sunday afternoon to discuss the nomination and to invite him to Maine, but did not formally offer the job. When Thomas arrived aboard an Air Force jet yesterday with Thornburgh, Gray and Sununu, Bush chatted with him alone in the bedroom of his residence for 15 or 20 minutes, then offered him the nomination.

They then joined the Bush family and aides for a lunch of crab salad and English muffins on a porch overlooking the Atlantic Ocean. Thomas tried to telephone his wife, but she was not in her office at the Labor Department. He reached her with the news just minutes before the news conference.

Bush told reporters Thomas met his requirement as someone who would "faithfully interpret the Constitution and avoid the tendency to legislate from the bench." He said he did not ask Thomas, who spent a year in a Roman Catholic seminary studying for the priesthood, his views on abortion rights.

The president praised Thomas as "a fiercely independent thinker with an excellent legal mind who believes passionately in equal opportunity for all Americans. He will approach the cases that come before the court with a commitment to deciding them fairly, as the facts and the law require."

Yang reported from Kennebunkport, Maine, and LaFraniere from Washington. Staff writers Helen Dewar and Ruth Marcus contributed to this report.