THE "Impeach Earl Warren" billboards seen throughout the Midwest, South, and West back in the 1950s and 1960s were puzzling. Those irate people who wanted the hide of Chief Justice Warren for his pro-civil-rights leadership on the high court didn't seem to be aware (or perhaps they forgot) that a man most of them almost revered, President Eisenhower, was responsible for elevating Warren to that lofty position. They never stopped asserting "I Like Ike," while he was in office or afterward.Warren, a former California governor, is one of the best examples of a Supreme Court appointee who moved in philosophical directions that had not been predicted. Warren was thought to be a conservative. Eisenhower assumed he was. Perhaps Warren thought he was, too. But in a position of independence where he could shape his own course without having to be reelected or reappointed, Warren's ponderings took him on an unexpected route toward becoming the champion of those who sought racial equality - not exactly a conservative cause back in those days. This brings us to Judge Clarence Thomas, President Bush's latest nominee to the Supreme Court. He's a black who has disdained the "politically correct" positions on racial matters as defined by most black leaders. He is an outspoken opponent of affirmative action and quotas. He thinks blacks, like members of other racial or ethnic groups, must lift themselves by their bootstraps. He feels he has done it this way himself - with good guidance, of course, which he acknowledges. This sounds like conservative dogma. It certainly mirrors the President's position. And this brings us to two observations: 1. Every voter in 1988 knew (or should have known) that what was at stake in the presidential election was the ability to make appointments that would shape the Supreme Court for the next generation. Liberals used every opportunity to warn the nation that a Bush win would mean the emergence of a rightward-veering court. That's what has happened. Associate Justice David Souter is showing early signs that he'll contribute to this shift. And now Judge Thomas appears likely to follow suit - if he is confirme d, which seems likely at this writing, though heavy drilling lies ahead. What else could or should the liberals expect? Wouldn't they have expected Michael Dukakis to have moved the court leftward had he made it to the White House? 2. The high court's independence from political pressure allows its members an element of unpredictability. There was the example of Warren, but one member of the present court, Associate Justice Byron White, would doubtless have surprised and maybe dismayed the liberal president who appointed him, John F. Kennedy. Edwin M. Yoder Jr., an expert observer of Supreme Court activity, describes Mr. White as "a judicial pragmatist of no predictable inclination or school of thought." Thomas, too, may show an independent spirit that will surprise his White House sponsor. Before the Senate hearings on his confirmation, Thomas hadn't aligned himself with either side on the emotional abortion issue. One hears speculation that his Roman Catholic background will lead him toward an anti-abortion stance. Also, a speech of his indicates that he might endorse this viewpoint. But he could surprise people. But let's not dwell too much on the uncertainties that may lie ahead for the new members of the court - or for the court itself. What seems certain is that the "conservative majority" that Presidents Nixon, Reagan, and now Bush have promised has finally come about. A big shift occurred last year when Associate Justice William Brennan, a liberal activist on the court, retired. And now that vocal guardian of the rights of minorities and the poor, Thurgood Marshall, has retired. A new, conservative era has dawned.