BOOK REVIEW / BIOGRAPHY : Wielding Power With a Clear Conscience : JUSTICE LEWIS F. POWELL, JR.: A Biography <i> by John C. Jeffries Jr.</i> ; Scribners $30, 690 pages

The nuclear trigger may make the President the most powerful person in the United States, but even the chief executive does not enjoy the profound and highly personal influence over our day-to-day lives that is wielded by the nine justices of the Supreme Court.

On so many questions that really count--abortion, capital punishment, civil rights, criminal justice, free speech and freedom of religion--five of the serving justices can make decisions from which there is no further appeal. And yet we know almost nothing about the men and women who serve on the high court.

So we come to “Justice Lewis F. Powell Jr.” a biography of the now-retired Nixon appointee who became the great centrist of constitutional law, with a certain curiosity. What will we learn from John C. Jeffries about the inner workings of the Supreme Court and the man whose swing vote made the difference in so many of its recent holdings?

Jeffries, a former clerk to Powell, is a law professor at the University of Virginia, and he approaches his subject with good taste and good manners, a meticulous attention to detail, and the sense of discretion that we might expect in a friendly but scholarly biographer.

At the end of these nearly 700 pages, we discover that Powell was a quiet but principled conservative who refused to march in lock-step with his fellow Nixon appointees--and thus made his mark on the often anonymous history of the Supreme Court.

Born in 1907 to a Virginia family with deep roots but not much money, Powell elevated himself by dint of hard work and what nowadays might be called networking. During his college years, Powell was befriended by a young Edward R. Murrow, who presciently nicknamed him “Judge,” and ended up in law school at Washington and Lee, then Harvard. His promising career as an attorney was interrupted by intelligence duties in World War II--but he returned to Virginia and resumed the practice that ultimately elevated him to a name partnership in a prominent law firm.

None of Powell’s life experiences are eye-opening or hair-raising, and we come to understand that Powell was a rather unremarkable gentleman lawyer with strong will, high principles, a sense of civic duty but not much passion.

His life appears considerably more momentous once we follow him to Washington, where we glimpse his initiation into the inner circle of the high court. Suddenly, the patrician lawyer from the Old South was invested with oblique but ultimately decisive authority, and charged with the task of making sense of a country that rocked on the edge of political crisis and social disorder.

Powell was selected by Nixon precisely because he appeared to be politically reliable. But, to Nixon’s surprise--and to the relief of more progressive court-watchers--Powell showed himself to be a principled conservative who did not hesitate to vote with the liberal minority. On the litmus-test questions of American politics--abortion, for example, and affirmative action--Powell quietly helped to rewrite constitutional law along humane and compassionate lines.

“Powell steadfastly supported the constitutionalization of abortion,” Jeffries writes. “Powell’s support for abortion tied into his core ideological commitment to the rule of law. If, above all else, people had to respect the law, it followed that the law had to respect their beliefs.”

Jeffries livens up the account now and then with tidbits that allow us to glimpse Powell in a somewhat more intimate light. Powell, an honorary member of the University Club in New York, was once rousted from bed and threatened with the bum’s rush by an officious clerk who declared that “he could find no record of membership, honorary or otherwise.” And, back home in Washington, the reserved and soft-spoken Powell ended up with the exuberant and effervescent Hubert Humphrey as his next-door neighbor.

“No one else,” Jeffries allows, “treated Powell ‘with quite the same informality.’ ”

The reader who pages through the book in search of sizzling scandals or dirty little secrets will be disappointed. Jeffries clearly cares less about Powell’s idiosyncrasies, which were few, than about the way in which Powell wielded his considerable authority.

“The history of Lewis Powell on the Supreme Court . . . is not a tale of strife and greed and conflict,” Jeffries writes, almost apologetically. “It is the story of the paradoxical protection of a democratic people by their least accountable and most elite institution.”