ON some key questions of legal rights - such as abortion and free speech - the conservative lock on the United States Supreme Court appears likely to become a little stronger next year.But the greatest significance of Supreme Court Justice Thurgood Marshall's retirement is likely to be on the character of the dialogue among justices, not the arithmetic of their decisions. Justice Marshall, the leading legal warhorse against racial segregation before he joined the court in 1967, brought a realism to Supreme Court deliberations about how rulings affected the poor and disenfranchised, according to court watchers. The remaining justices "have no sense of what it's like to live in a poor part of town and how people there might relate to authority, notes Louis Michael Seidman, a Georgetown University law professor and former clerk for Justice Marshall. "There was a sort of man-of-the-people earthiness to Justice Marshall's view of life and the law," notes Rodney Smolla, a law professor at William and Mary College in Williamsburg, Va. In practical terms, the Supreme Court's conservative bloc has already consolidated its majority in most major areas of the law - often with 6 votes to 3 in divisive cases. Its effort to roll back some of the great expansions of individual rights under the Warren court in the 1960s is well under way. During the term just ended, the most dramatic shift in the court's position was in scaling back the rights of the accused and restraints on police. Some of the changes: * A condemned prisoner can be barred from pursuing a challenge to his sentence, even if it concerns his guilt or innocence, because of a procedural error by his attorney. In Coleman v. Thompson, a prisoner's petition to federal courts was denied because his attorney missed a state filing deadline by three days. * A coerced confession remains inadmissible evidence in court, but even if it is admitted during the trial, it can be ruled "harmless error" and the conviction sustained. * From several cases: An officer can give chase without any probable cause or reasonable suspicion of wrongdoing. Once an officer decides he has probable cause to search a car, he can open up closed packages as well without a search warrant. An officer can search luggage on board a bus, with passenger consent, without a warrant or probable cause. * A life sentence without parole for possession of about a pound and a half of cocaine, a mandatory sentence under Michigan law, does not violate the constitutional ban on cruel and unusual punishment. The Supreme Court this year, says Professor Smolla, "has really taken the federal courts out of the business of reviewing state criminal convictions. So you're not going to get a second bite at the apple" if convicted of a crime. The newest justice, David Souter, has yet to clearly define his position on the court, but in gerneral he appears to fall about in the middle of the court ideologically. The court had only a flyby at the abortion issue this term. Rust v. Sullivan, barring medical clinics receiving federal funds from mentioning abortion services to patients, was more of a free-speech case. That is likely to change next term. At least three jurisdictions - Louisiana, Utah, and Guam - have enacted laws prohibiting abortions under most circumstances. The laws directly challenge the 1973 Roe v. Wade decision that a constitutional right to privacy includes the right to an abortion. One of them seems bound to provide a straightforward test of abortion-rights in the Supreme Court next year. Most legal observers have been expecting the court to overturn Roe v. Wade on the first opportunity, but only four justices are clearly in that camp. "They might have found a fifth vote before," says David Strauss, a University of Chicago law professor. With Marshall virtually certain to be replaced by a conservative appointee, they will have that vote, says Professor Strauss. The greater certainty of a majority may even make it a broader majority, since a wavering justice might be more comfortable joining a 7-to-2 decision than casting a swing vote. Justice Sandra Day O'Connor in particular has come down on either side of abortion-related questions, and court watchers have long speculated that she does not want to cast a tie-breaking vote on the issue. The loss of Marshall's consistent liberalism on the court could be critical to free-speech rights. The court ruled that flag-burning is constitutionally protected speech in 1989 with a 5-to-4 vote. When Marshall steps down, only three of those five majority votes will remain. Another area where the observers are eager to see how far the court moves is on the First Amendment ban on establishment of religion. One test in the legal pipeline for Supreme Court consideration is a Providence, R.I., challenge to the religious benediction at a public-school graduation. But the more significant issue is how the courts allow tax dollars to be used in church-affiliated schools. No obvious cases have yet arisen.