President Reagan's selection of Justice William H. Rehnquist to be chief justice could turn out to be the best thing that has happened to liberal Justice William J. Brennan Jr. since Justice Harry A. Blackmun's alleged conversion to liberalism.

Rehnquist's almost certain confirmation may ultimately give Brennan more power. As the court's senior member, Brennan may find himself controlling -- through the power of assignment -- more of the court's opinions than in the past.

That ironic outcome is possible, court insiders say, because Rehnquist's conservative views put him on the right wing of a court that continues to be dominated by a centrist group of justices.

While both Rehnquist and appeals Judge Antonin Scalia, who has been named to replace him, are significantly more conservative than the retiring Burger, the changes will not greatly affect the court's political lineup. That lineup has Brennan and Justice Thurgood Marshall holding down the liberal wing with Rehnquist and Burger on the right, and each side vying for the votes of those between.

Ceremonial trappings aside, the only exclusive power that the chief justice has is the right, when in the majority, to assign who writes the opinion for the court. But that power is significant.

An opinion describes not only the final result, but the legal path the court takes to reach it, thereby establishing guidelines to be used by lower court judges in future cases. The tone, the clarity and the reasoning outlined by the author often are more important than the outcome of the case being decided.

Burger for years was accused of using a "bait-and-switch technique" to control the opinion-writing assignment, even when he was on the losing side. According to well-informed sources, Burger often would pass rather than speak first, as the chief justice is expected to do during the justices' secret conference in which a tentative vote is taken on a case. The result might be a 5-to-3 vote, with Brennan the senior justice in the majority.

Burger then would vote with the majority, assign the opinion to the most conservative justice in the majority and concur in the result only -- or sometimes join the dissent. Even in cases in which he voted first, he sometimes switched at the end of the conference in order to make the assignment.

A moderate to liberal result would prevail, but the majority opinion would usually be far more conservative than if Brennan had been allowed to write it or to assign it to a liberal member of the majority.

Insiders say it is inconceivable that Rehnquist would pass on the first vote or use sleight-of-hand tactics to stay in the majority. Rehnquist, say these sources, would state his views in the case and vote. Given those views and the current makeup of the court, Rehnquist would be in the minority far more often than Burger was.

In addition, Burger felt as chief justice that it was important that he not dissent too often -- and especially that he not dissent alone.

Rehnquist, in contrast, has dissented alone more than any other justice on the court, according to a study by the weekly National Law Journal. Those dissents, 54 in all, have come in major civil rights, First Amendment and criminal cases.

Rehnquist dissented alone, for example, when Burger wrote for the rest of the court that racially discriminatory private religious schools may be denied tax exemptions. He dissented alone when the court declared it unconstitutional for a state to presume that pregnant women are unable to work, when the justices said states may not automatically exempt women from jury duty and when they held that a state may not discipline a lawyer for recruiting members for the American Civil Liberties Union.

It is possible that Rehnquist, once he becomes chief, will decide that he must try to move slightly toward the center to lead the court. He recently has joined (Wygant v. Jackson Board of Education) or written (Meritor Savings Bank v. Vinson) opinions that some observers say indicate a new flexibility of views.

But his 15 years of staking out definitive and rarely altered positions make it difficult for him to depart significantly from that conservative judicial philosophy.

He also has not shown an inclination to "give a little, get a little" to mold a conservative majority. Rehnquist has never walked the halls to muster votes, and few who know him think he will start now.

Other justices have been known to cast an initial, tentative vote and then join a majority opinion or a dissent by writing a "You have persuaded me . . . " note to their colleagues. One insider said he had never seen or heard of Rehnquist sending such a note.

If he does not alter his views and is not able to muster a majority for those views, Rehnquist, even with the addition of conservative hardliner Scalia, may remain relatively alone in dissent.

The Rehnquist-Scalia wing will bring great intellectual powers of persuasion to the right in battling the liberals for the centrists on the court. But unless one of the court's moderates or liberals leaves -- and none has indicated any intention to do so -- it seems likely that Rehnquist will wield the assignment power far less than Burger did.

The shift so hailed by conservatives and so lamented by liberals may turn out, at least in the short run, to surprise them all. Such are the prospects for the Rehnquist court. CAPTION: Picture, Justice Brennan, a leading liberal, is likely to assign majority opinions more often in a court led by Justice Rehnquist. (c) KEN HEINEN