WASHINGTON (AP) _ President Bush's first Supreme Court appointee likely will strengthen an already-dominant conservative majority that no longer need battle William J. Brennan's liberal leadership.

The result: a rightward-moving Supreme Court is expected to grow even more conservative when addressing such issues as abortion, civil rights and criminal justice.

Speaking a month before Brennan's retirement Friday, Justice Harry A. Blackmun predicted the inevitable departure of the court's three liberal octogenarians - himself, Brennan and Justice Thurgood Marshall - would leave conservatives in control for half a century.

''We shouldn't resent it,'' Blackmun told The Associated Press. ''That's the way the system works. That pendulum swings. It will stay this way now 40 or 50 years, I'm sure.''

The impact of Brennan's retirement, in part, is purely mathematical. The president's stated intention to pick a ''strict constructionist'' means six rather than five conservative votes among the nine justices.

But Brennan's departure may have a more profound impact, one that goes to the personal dynamics of the court as a political institution.

Combining keen intellect and charismatic charm, Brennan was a master politician and coalition builder who for years blunted conservative gains.

Brennan's use of his law clerks as emissaries to the offices of his fellow justices was a behind-the-scenes fixture as well known as his memos to fence- sitting court members.

In recent years, if one or more of the court's conservative justices - Byron R. White, Sandra Day O'Connor, Antonin Scalia or Anthony M. Kennedy - could be attracted to the liberal side of an issue, it was Brennan doing the wooing.

None of the three remaining liberals - Blackmun, Marshall or Justice John Paul Stevens - can assume that role easily.

''No one will fill those shoes,'' said A.E. Dick Howard, a University of Virginia law professor.

Bruce Fein, a conservative court analyst, predicted the court without Brennan will become ''a conservative juggernaut.''

''The remaining liberals will become dispirited. They just don't have the strength and winsome character to bring up that fifth vote,'' Fein said.

Although less successful against Chief Justice William H. Rehnquist's conservative leadership since 1986, Brennan still could claim some victories.

In the 1989-90 term that ended in June, Brennan led the court as it upheld a federal affirmative action plan giving minorities special consideration in awarding television and radio licenses and struck down a ban on flag burning. Both cases were decided by 5-4 votes.

The affirmative action ruling surprised and delighted many civil rights activists who in recent years have criticized the court's decisions making racial and sexual discrimination more difficult to prove.

Brennan, who joined the court in 1956, was a leader of the liberal majority that through the 1960s expanded individual rights and mandated greater equality for the nation's minorities.

But more recently an increasingly conservative court has curtailed some of the legal doctrines he fashioned, particularly those that limited police powers.

Brennan's successor will join a court now more likely to give greater weight to the frustrations of law enforcement officials than to the asserted rights of those suspected of crime.

No legal battleground is more closely or fiercely contested at the high court than that of abortion. And any new court appointee is sure to have an impact on the future of abortion in the United States.

There appear to be four votes - Rehnquist, White, Scalia and Kennedy - to overturn the court's 1973 ruling in Roe vs. Wade that established a constitutional right to abortion.

There also appear to be four votes opposing such a move. Blackmun, Marshall and Stevens are strong supporters of abortion rights. And the generally conservative O'Connor has refused so far to cast the fifth vote to reverse Roe vs. Wade and let states outlaw abortion.