POLITICALLY, the Bush appointment of Clarence Thomas to the Supreme Court was very shrewd. Black leaders wanted another Thurgood Marshall. But they are finding it difficult to vigorously oppose a man who doesn't mirror Justice Marshall's views on civil rights.Why? Because Mr. Thomas is not only black but he has knows what its like to start life in a sharecropper's shack. Many poorer blacks could regard him as one of their own. Little wonder, then, that polls show a strong majority of American blacks favoring confirmation of the Thomas nomination while at the same time disapproving of his outspoken opposition to affirmative action. Washington Post columnist William Raspberry, himself a black who would prefer a Marshall-style liberal on the nation's highest bench, offers this insight into the dilemma that Thomas's nomination poses for blacks: "We're playing 'Let's Make a Deal' with a host who offers us a choice between a serviceable Chevrolet and a goat, and we're holding out for a curtain that conceals (we hope) a Mercedes Benz with an interior designed by Thurgood Marshall. Well, there's no Benz behind any of the curtains. If we're not prepared to deal with the goat, we'd better take the Chevy." Raspberry thinks Thomas "is sufficiently acquainted with racism to recognize it when it comes before him on the Supreme Court, that he is independent enough not to see the critical issues in the light of his own experience and that he is smart enough to find in the Constitution protection against the presumptions of white privilege." Raspberry is concerned, as are many other influential blacks, that should Thomas's nomination be turned back the next Bush appointee might be a white who would not only be conservative but also less sensitive to black problems. I would argue that such an appointment might, like the Earl Warren selection by President Eisenhower, turn out to be surprisingly pro-black. But black leaders are leery about taking such a chance. Raspberry's view, too, includes a certain amount of appreciation for Thomas's emphasis on blacks having to make it on their own. "I hear him saying with [Virginia Gov. Douglas] Wilder that blacks are foolish to wait for whites to deliver us, that we must return to the old values that worked for us in harsher times than these...." So, at least in these early days following the selection of Thomas to the high court, opponents have been thrown off balance. Mr. Bush doubtless believes he made a high-quality appointment. But superb politician that he is, the president must be credited with being fully aware that by selecting a black he would cause disarray in liberal ranks. Bush would have had to know that liberals in general and blacks in particular would get his implicit message: You turn down Clarence Thomas and this may be your last chance to have a black on the United States Supreme Court for years to come. The thought probably crossed the president's mind, too, that a lot of blacks would be unhappy with the politicians who derailed Thomas - should that come about. The members of the Senate Judiciary Committee - all of them white - would probably anticipate that rejection of a black, even a conservative black, might antagonize nonwhite voters around the country. They would be fearful of appearing to pick on a black nominee. Bush also, even before selecting Thomas, was doing quite well among blacks. Polls show that about 50 percent of blacks nationwide currently approve his performance. For a Republican president that's an unusually high rating. This is another factor making it difficult for civil-rights leaders to persuade blacks to unite behind a stop-Thomas move. Similarly, pro-choice leaders may not find it easy to launch a strong, effective campaign against Thomas, whose background and past speeches indicate a leaning against abortion. Polls show women backing Bush in almost the same extremely high proportions as men.