WASHINGTON (AP) _ Lewis F. Powell Jr. leaves behind a legacy of major decisions and a Supreme Court ready to tip in the ideological direction of his successor.

In many of the cases decided during his more than 15 years on the nation's highest court, Powell was like a man in the middle of a teeter-totter. However he leaned, the law would move.

In April, for example, the court handed opponents of capital punishment a major defeat by ruling that state death penalty laws are constitutional even when statistics indicate they were applied in ways that are racially biased.

The opinion, which upheld Georgia's death penalty law, was by a vote of 5-4. The vote that tipped the scale was Lewis Powell's.

''Apparent disparities in sentencing are an inevitable part of our criminal justice system,'' Powell wrote for the majority. ''Where the discretion that is fundamental to our criminal process is involved, we decline to assume that what is unexplained is invidious.''

The American Civil Liberties Union, in commenting on Powell's resignation announcement on Friday, said ''in 20 important civil liberties cases decided by one vote this term, Justice Powell was the deciding vote in each instance.''

The Georgia case, which upheld the death sentence given to Warren McCleskey for the 1978 murder of an Atlanta policeman, was one of those in which Powell's centrist stance went against civil liberties advocates.

Bruce Fein, a visiting fellow for constitutional studies at the Heritage Foundation, also said that Powell's was the pivotal vote on a vast majority of contentious civil rights cases.

''Powell's vote enabled state governments and the private sector to grant gender and racial preferences without proof of past illegal discrimination,'' said Fein. ''He was instrumental in keeping Roe v. Wade, the landmark abortion decision, intact. He was repeatedly important in church-state relations cases and in curtailing any type of government finacial aid to non-public schools.''

Powell voted frequently to curb use of the death penalty, and he was pivotal in a ruling two weeks ago that statements from the family of a murder victim cannot be used in capital sentencing proceedings.

''One can understand the grief and anger of the family caused by the brutal murders in this case,'' Powell wrote for the 5-4 majority. ''But the formal presentation of this information by the state can serve no other purpose than to inflame the jury and divert it from deciding the case on the relevant evidence concerning the crime and the defendant.''

The jury's task is to decide ''whether the death penalty is appropriate in light of the background and record of the accused and the particular circumstances of the crime,'' Powell said.

In 1982, the justices voted 5-4 to overturn the death sentence of a young man convicted of killing an Oklahoma state trooper. The court left unanswered the major question it had been expected to raise in the case, whether convicted murderers who commit their crimes when under age 18 can be sentenced to death.

Powell's opinion said evidence of a convicted killer's family history and emotional problems need not be given much weight, but he added:

''When the defendant is 16 years old at the time of the offense, there can be no doubt that evidence of a turbulent family history, beatings by a harsh father, and of severe emotional disturbance is particularly relevant.''

Last year, the court gave a blow to affirmative action in the American workplace by striking down a plan aimed at protecting the jobs of black school teachers in Jackson, Mich., at the expense of whites with more seniority.

Powell was on the side that said the affirmative action plan violated the Constitution's guarantee of equal protection for all.

''This court has never held that societal discrimination alone is sufficient to justify a racial classification,'' he wrote for the 5-4 majority. ''Rather, the court has insisted upon some showing of prior discrimination by the governmental unit involved before allowing limited use of racial classifications in order to remedy such discrimination.''

In 1983, Powell was on the side that found a life sentence without possibility of parole for ''relatively minor cirminal conduct'' such as passing bad checks amounts to cruel and unusual punishment.

Powell wrote that a life sentence for a man convicted of writing a phony $100 check ''is significantly disproportionate to his crime and is therefore prohibited by the 8th Amendment.''

He also wrote a 5-4 opinion that labor unions may not discipline members who quit during a strike and go back to work.

''The congressional purpose to preserve unions' control over their own internal affairs does not suggest an intetnt to authorize restrictions on the right to resign,'' he wrote. That brought a strong dissent from Justice Harry A. Blackmun, who called the ruling ''an affront to the autonomy of the American worker.''