The Supreme Court term that ended Friday underscored the ability of Justice Lewis F. Powell Jr. to push the court toward the centrist philosophy that characterized his 15 1/2-year judicial career.

Powell's diligent effort to seek the middle ground, deciding each issue on a case-by-case basis, made him a critical vote to both wings of the court. Liberals will miss the retiring justice for his votes on social issues, while conservatives lost a generally reliable ally on most criminal and business issues.

Powell was on the winning side nearly 90 percent of the time in this term's 147 opinions, far more than any other justice. More important, in the 42 most divisive cases, where the court split 5 to 4, Powell was on the winning side in all but eight.

Powell's vote enabled liberals to win clear majorities in several of the most controversial cases of the term, involving affirmative action, the use of victim-impact statements in death penalty sentences and the First Amendment rights of public employes.

But Powell sided with the conservative wing of the court 81 percent of the time overall, and 75 percent of the time in those 5-to-4 cases.

If President Reagan succeeds in appointing a solid conservative to replace Powell, the court's direction may show little change in cases involving criminal justice, business, labor or federalism.

The difference, and it could be major, will likely be in cases involving civil rights, civil liberties and abortion, where Powell often gave liberals a decisive vote.

The first term of the court under conservative Chief Justice William H. Rehnquist was marked by a series of unexpected liberal victories in controversial cases. But liberals, led by Justice William J. Brennan Jr., are likely to find it difficult to repeat that performance without Powell's help.

Powell's vote, however, was not crucial in all the liberals' victories. The moderate-liberal wing was often supported by one or more of the court's conservatives. One major victory for the liberals on affirmative action, for example, attracted a concurring vote from Justice Sandra Day O'Connor. O'Connor, who sided with Rehnquist 83 percent of the time, also was on the liberals' side in controversial cases involving pregnancy-disability leave, protections for persons with diseases such as acquired immune deficiency syndrome (AIDS), and teaching creationism in the public schools.

The newest justice, Antonin Scalia, after what conservatives felt was a shaky start, emerged as a solid member of the conservative wing. Even so, Scalia, who voted with Rehnquist nearly 87 percent of the time, cast the decisive vote for the liberals in two significant criminal cases and sided with them in several other important decisions, including the pregnancy-disability ruling.

The fourth solid conservative, Justice Byron R. White, who supported Rehnquist 84 percent of the time, also parted company in a voting rights case, a housing rights case and a free speech case, along with the creationism and AIDS cases.

This term even saw a few so-called liberal victories decided on unanimous votes. These included a decision allowing Arabs and Jews to use a federal civil rights statute that some argued was intended only to help blacks, one that involved admitting women into "male-only" clubs, and a third that struck down an ordinance banning all "First Amendment activities" in a public airport. As a rule, about 25 percent of the court's cases are unanimous.

Conservatives won a series of major triumphs in their traditional area of strength in the last 15 years, criminal law. The court upheld the constitutionality of preventive detention and, with Powell providing the fifth vote, rejected a major challenge to the death penalty. Powell was also in the five-member conservative majority overturning a liberal precedent barring courts-martial of military personnel for crimes not connected with their military service.

But conservatives also lost some criminal cases, including an opinion sharply curtailing a mail-fraud statute heavily relied on by prosecutors in cases of state and local government corruption. In another conservative loss, with Powell giving liberals the fifth vote, the court struck down the use of "victim impact" statements in cases involving the death penalty.

The death penalty appears to involve what is almost a separate category of criminal law, with the court often saying that "death is different" from all other punishments and merits special judicial safeguards to ensure fairness.

In the criminal area, the court this term gave police and other officials more authority to conduct warrantless searches in some places, such as government offices, junkyards and homes of people on probation. But in other cases the court seemed willing to hold law enforcement officials in check, refusing to relax the probable-cause and warrant requirements for traditional police investigative searches.

In several cases, the court continued to apply the 1966 Miranda decision -- requiring police to warn suspects of their rights -- but did not expand on it.

State and local prosecutors, for reasons not entirely clear, seemed to have had a harder time this term getting the court to agree to hear their appeals. The court agreed to hear only 10 percent of the states' appeals in criminal cases, which appears to be a lower percentage than in the past. At the same time, the justices agreed to hear more appeals from defendants, including several granted over the opposition of the solicitor general.

Prosecutors still win a substantial majority of their cases, but their dominance appears to have diminished.

One explanation may be that Scalia is not as reliable a vote for prosecutors as former chief justice Warren E. Burger. Scalia, in one search case, rejected a position set out by Rehnquist and joined by Burger in a prior ruling. This time, Scalia wrote the 5-to-4 ruling upholding the defendant's claim.

The Reagan administration, rocked by a series of bitter defeats last term and early in this term, managed to win 67 percent of its cases, down slightly from its 72 percent winning rate last year and substantially down from its extraordinary 87 percent victory rate in 1984.

The administration lost several major cases, especially in the civil rights area, even though it seemed to moderate its hard-line conservative views in a number of areas and steered clear of involvement in several controversial cases, such as creationism.

The retirement of Powell, who cast key votes against the administration in several of its losses, gives the administration the opportunity to reverse or at least mitigate some of those losses in future terms.

That is the biggest worry to civil rights groups, especially those involved in women's issues. As a result of Powell's resignation, the "civil rights community as a whole is vulnerable," said Marsha Levick of the legal defense fund of the National Organization for Women. "For the particular issues that we are concerned about," she said, "we alone are very vulnerable."

Next term the court is scheduled to hear a case challenging the constitutionality of a state law requiring a waiting period and parental notification before a minor can have an abortion. "That case would have been difficult" for the pro-choice groups to win "under the best of circumstances," Levick said. "Now it will be worse."