AFTER two months during which Clarence Thomas's fitness to be a justice of the Supreme Court has been debated in the press, it's a relief to have the confirmation hearings commence in the Senate Judiciary Committee. Finally we'll get to take the measure of the nominee, who, in keeping with tradition, has kept his own counsel since he was tapped by President Bush to succeed Thurgood Marshall on the high court.At present it appears that Mr. Thomas will pass muster with the Senate, though it's not likely the vote to confirm will come in time for the beginning of the court's term next month. A coalition similar to the bloc that defeated Robert Bork's nomination in 1987 has mobilized against Thomas, but its efforts to depict Thomas as outside the constitutional mainstream haven't resonated to the same degree. Although we had grave misgivings about Bork's views, we're not eager for a repeat of the angry atmosphere of those hearings, where caricatures and smears sometimes impinged on thoughtful and legitimate criticisms of the nominee's record. The Thomas hearings should be conducted with fairness, dignity, and respectfulness on all sides. That doesn't mean, however, that the senators should skirt hard issues. The time is gone when a Supreme Court nominee could decline to answer questions about his or her judicial philosophy. Without compromising judicial independence or prejudging matters that may come before him on the bench, Thomas can reasonably be expected to articulate his approach to judging, his views on the nation's constitutional order and heritage, and his vision of the great issues of civil and human rights that permeate the mo ral and social climate of our times. Inevitably, the hearings will get down to questions about Thomas's views on civil rights, affirmative action, privacy, and abortion. We anticipate having differences with the nominee on some of these issues. Yet single-issue "litmus tests whether used by judicial screeners in the White House or by critics of nominees to the high court - should be avoided. What ought to be looked for in Judge Thomas's testimony is not congenial doctrine so much as evidence of open-mindedness, moral imagination, and sensitivity to the real-life problems of people who, for reasons of race, gender, or poverty, are powerless or face blighted prospects. Nothing less is acceptable for the successor to Thurgood Marshall.