CLARENCE THOMAS makes a provocative possible successor to United States Supreme Court Justice Thurgood Marshall.Even before President Bush named him, Judge Thomas appeared to be whom Justice Marshall had in mind when he worried about the president replacing him with "the wrong Negro." For those concerned that, with Marshall's departure, the court is losing its only present member with firsthand knowledge of racial oppression and life among the poor, Thomas would seem to fill the bill. "I would think there would be a tremendous impulse [for him] to identify with the poor," notes Abraham Goldstein, who taught Thomas criminal procedure at Yale Law School. Yet Thomas is also a conservative who believes that the modern civil rights movement began going off the track even in its founding victories - such as the Brown v. Board of Education decision in 1954 that mandated desegregating public schools. Departing Justice Marshall was the legal mastermind of the modern civil rights movement and personally argued the Brown case before the Supreme Court. The confirmation battle in the Senate will begin in September. The signs are already out that Thomas will not have an easy time. Sen. Paul Simon (D) of Illinois, a member of the Judiciary Committee, which holds hearings and initially recommends whether or not to confirm judicial appointments, immediately declared that Thomas's views, not his background, matter most and charged him with "going out of his way" to weaken civil rights protections as chairman of the Equal Employment Opportunity Commission. An immediate concern of many senators is to ascertain Thomas's views on abortion, an issue likely to come before the Supreme Court in a very direct form during its next term. Unlike David Souter, the little-known and still little-understood Supreme Court justice from New Hampshire, Clarence Thomas has been a prolific writer throughout the 1980s. Most of his published views concern discrimination and civil rights. He shares the view that dominated the Reagan administration: that the law should be colorblind. Thomas is strongly opposed to racial segregation, but in a 1987 law review article he explained how he believed the civil rights revolution went slightly wrong. He says that in the Brown v. Board of Education decision, Chief Justice Earl Warren made the right decision - to desegregate public schools - for the wrong reason. Justice Warren's opinion rested on sociology, on racial sensitivity, and on the sense of inferiority that racial segregation promoted. Thomas writes that segregation is better fought as an injustice rooted in slavery that defies the promises of liberty in a "colorblind Constitution." The difference in the reasoning lies in its respect for man's ability to reason and choose, as Thomas explains it. On other issues, his views are less developed, at least in public forums. He has been a judge on the United States Court of Appeals in the District of Columbia for about 15 months, writing a little more than a dozen opinions. Pepperdine University law professor Bernard James, who has studied them, sees Thomas as a part of a certain movement of conservatives percolating up through the judiciary. Mr. James calls them the "reasonable basis" judges - willing to keep the courts from interfering with government power and the will of the majority as long as there is a reasonable basis for the government action. Yet this standard leaves some room for judicial activism, he says. For example, Thomas has been a strict conservative in upholding criminal convictions against challenges to evidence used against those convicted. On abortion, the clues are very general. The Roe v. Wade decision that threw out all laws banning abortions is one of the most extreme cases of judicial activism, and Thomas does appear to favor judicial activism. He also grew up in a devout Roman Catholic family and attended Catholic schools through college - including one year at a seminary - before he "decided not to become a priest." Although Catholics' views on abortion are highly varied, the church itself is strongly opposed to it. Thomas grew up poor near Savannah, Ga. He is now financially comfortable, but by no means wealthy. Based on the financial statement he filed for confirmation to the appeals bench a year and a half ago, he had total assets of $281,000 and owed $91,000 - not the affluence expected of a prominent, widely published, Yale-educated lawyer. "This fellow has some life experiences that may lead him in other than a conservative direction," says James.