THE Supreme Court - prime mover in the nation's vast midcentury expansion of legal rights - today opens the next chapter of its conservative counterrevolution.The clearest chances for landmark shifts in legal doctrine during the 1991 session, according to an array of court-watchers, are cases that will give the tribunal opportunities to define free-speech rights and the ban on the establishment of religion. On other fronts, the counterrevolution, as conservative legal scholar Bruce Fein calls it, is virtually over. By the end of the court's last session it had pruned many of the safeguards for accused or suspected criminals that were expanded by the Warren court in the 1960s. The year before, in the 1989 session, the court dramatically narrowed its protection of the free exercise of religion. The question some lawyers and scholars ask now is: What kind of conservatism will carry the court - judicial restraint or conservative activism? Will the court continue to narrow its power in deference to legislators and government agencies? Or will it flex its muscle to protect conservative ends - such as property and contract rights - in much the way the Warren court asserted the rights of women and ethnic minorities? As the 1991 session begins today, no liberal justice is left on the bench. Justice Thurgood Marshall, the lawyer who argued and won the Brown v. Board of Education school-integration case in 1954, formally stepped down last Tuesday. The first case the court hears this morning involves school desegregation, where the justices may further narrow the legacy of that case. The two Bush appointees to the court remain largely unknown. Judge Clarence Thomas has not been confirmed yet, although sufficient votes are lining up behind him so that he is likely to join the court shortly. Justice David Souter has not yet made a clear imprint. Legal scholars suspect he is not leading any conservative charges on the court, but his opinions have been strongly pro-law enforcement concerning criminal law and procedure. On the most politically loaded issue before the court - abortion rights - some direct-hit cases are on the way up through the system. Louisiana, Utah, and Guam have banned abortion in direct assaults on the 1973 Roe v. Wade decision that guaranteed abortion as part of a constitutional right to privacy. Laws in Pennsylvania and Missouri have placed restrictions on abortion that fall short of outright bans. So far, however, none of these cases is on the docket for this session, and they may not make it until next year. The closest to an abortion case on the agenda is Bray v. Alexandria Women's Health Clinic, a suit against Operation Rescue, the anti-abortion activist group, for blockading abortion clinics in the Washington, D.C., area. The suit uses a Civil War-era law aimed at the Ku Klux Klan to charge Operation Rescue with conspiring to deprive pregnant women, as a class, of their right to travel freely. This case is not likely to pivot around the right to an abortion. But if the group-bias argument works against Operation Rescue, then many other statutes that treat abortion specially may be in jeopardy. The case that appears to present the most sweeping potential for a change of legal doctrine concerns the establishment of religion. Lee v. Weisman, a challenge to a prayer offered at a Providence, R.I., middle-school graduation, "invites restructuring of the establishment clause" of the Constitution, says Mark Tushnet, a Georgetown University law professor. The current standard calls for strict government neutrality toward denominations and religion in general but allows for a modest ceremonial role for religious expression. Some justices, however, particularly Antonin Scalia, may be tending toward barring only religious coercion. If this view carries the court, then prayer in public schools may get new consideration. The court is not moving in any clear direction on free-speech questions. It will have some cases this year where defining free-speech is directly at issue. One is Simon & Schuster v. New York Crime Victims, in which the publishing house is fighting a New York law that bars criminals from profiting from books and movies about their crimes. On one hand, the law targets speech according to its content, which would appear to chill free speech. On the other, it only goes after the criminal's share of the profits; it does not forbid the book or movie. Many states have these laws, but New York's is one of the broadest, so it could be struck down in a way that leaves others standing. Another case concerns racist speech. In R. A. V. v. St. Paul, a youth was convicted of burning a cross in a black family's yard. An ordinance bars such actions when they will arouse "anger, alarm, or resentment in others on the basis of race, color, creed, religion, or gender." He argues the ordinance is too broad and could be used against constitutionally protected speech. The obligation of school districts to desegregate their schools may be scaled back in Freeman v. Pitts, the first case of the session. This is the clearest opportunity in recent years for the court to determine just when a school district can be released from court control. In it, DeKalb County near Atlanta has complied with a court order requiring neighborhood schools, but the racial makeup of neighborhoods has changed so dramatically that high schools are all at least 90 percent single-race. The basic legal question is whether a district can meet its obligations to desegregate schools and be released from court orders one at a time. Another case raises fairly mundane legal questions but with huge potential dollar impact. Cipollone v. Liggett Group is a case of a widower of a deceased smoker seeking damages from a tobacco company. The question is whether the health warnings on cigarette packages, in accordance with federal law, override any state laws holding the tobacco companies liable. If the federal requirements don't trump in this case, it could open the door to vast numbers of liability suits against tobacco companies and other businesses.