THE RESIGNATION of Justice Lewis Powell is the most important change in the composition of the Supreme Court in 15 years. The soft-spoken, widely respected justice leaves an intellectually divided court, where he was often the critical man in the middle. President Reagan's earlier appointment of Justice O'Connor and Justice Scalia and his elevation of Chief Justice Rehnquist caused little shift in the balance of power on the panel. Justice Powell has more often been found in the exact center of the court, particularly in cases involving social issues. This term he has been with the chief justice 70 percent of the time in 5-4 cases, but on important issues he has often surprised those who sought to label him politically.

Since October, according to the ACLU, 20 civil liberties cases have been decided by a single vote: Justice Powell's. His decisions haven't always pleased liberals, but a good part of the time the Reagan administration was disappointed too. He voted to strike down state laws restricting the right to abortion; he opposed public funding of church schools; and he sanctioned affirmative action plans, even some of the most far-reaching controversial plans that have come before the court in recent years.

Without his vote, the court's direction in these areas becomes uncertain. So too in criminal law. The Reagan agenda calls for overturning the Miranda rule and giving the police much greater powers to search and seize. While Justice Powell usually supported prosecutors, most observers thought he would not go so far as to abolish important protections for the accused. And though he voted to sustain the death penalty, he has been one of the justices who view capital punishment as so extraordinary as to require special procedural safeguards limiting its applicability.

Praise for Justice Powell has been non-partisan. He proved to be a jurist of intellect and honesty. He came to the court with a reputation as a southern gentleman, a civic-minded citizen and an excellent lawyer. He had been an important figure in his state and the president of the American Bar Association.

The White House would do well to look to his example in choosing a successor. Justice Powell was himself appointed to the court by a president who had failed to win Senate confirmation for two successive nominees after blistering, politically charged battles. The potential for such a fight exists now, but it can probably be avoided if the president chooses a nominee of integrity and scholarly qualification, a candidate the Senate will find impossible to thwart, even if some there are politically so inclined.