WASHINGTON — The start of another Supreme Court term next Monday reminds us that the most influential single public official in this land may not be anyone in elective office, but Justice Sandra Day O’Connor.

Holding the swing vote on the nine-person Supreme Court, she can and does set more policy than President Bush or all 100 members of the Senate, 435 representatives or 50 governors.

Her special role was exemplified when the court last month assembled a month ahead of schedule in order to hear four hours of oral arguments on the McCain-Feingold campaign finance law. The advocates on both sides, representing key figures in both the executive and legislative branches, directed most of their efforts toward O’Connor, knowing that she is probably the one whose vote will swing the case.

The heavy weight this 73-year-old Arizonan carries stems in part from the ideological balance of the other eight jurists, with four to her right and four to her left. But it also reflects the skill and tact with which she has fortified her position in her 22 years on the high court.

O’Connor is a great one for finding the center, not just of legal disputes but of the political spectrum. The only justice who previously held elective office — as Republican majority leader of the Arizona state Senate — she can navigate her way through the most ticklish of situations, even if it requires splitting the difference.

She did that last term in the celebrated University of Michigan affirmative action case. O’Connor wrote the 5-4 majority opinion upholding the principle that racial diversity is an important value in higher education and therefore can be a factor in deciding admission to law school. But then she turned around and joined a different coalition striking down the mathematical point system Michigan used to rate applicants to its undergraduate student body.

O’Connor is not predictably an ally of the court’s liberal bloc or its opposition. In two 5-4 decisions, she wrote opinions upholding the application of California’s "three strikes" law — one of the toughest criminal statutes in the land.

In one case, the defendant was sentenced to life in prison, with no possibility of parole, after stealing $153 worth of videotapes from a Kmart. In the other case, the crime that triggered a similar punishment was theft of three golf clubs valued at $1,200.

Defense counsel argued that lifetime imprisonment for such minor crimes constituted "cruel and unusual punishment" forbidden by the Constitution. But O’Connor ruled that California was within its rights to insist that a pattern of criminal behavior justified such a punishment and nothing in the Constitution forbade it.

Over the years, she has almost singlehandedly constructed the code that legislatures must follow for redistricting plans to survive judicial scrutiny.

An essay by Erwin Chemerinsky, a professor of law at the University of Southern California, in the latest issue of The Green Bag, aptly described as "an entertaining journal of law" by its publisher, George Mason University, focuses on O’Connor’s extraordinary role in many other areas of law.

"The October 2002 Supreme Court term ended dramatically," he wrote. "In the last week of June, the Supreme Court upheld affirmative action programs by colleges and universities; invalidated a state law prohibiting private consensual homosexual activity; overturned a death sentence because of ineffective assistance of counsel; upheld a federal law requiring libraries that received federal funds to install Internet filters; and declared unconstitutional a California law that retroactively extended the statute of limitations for sex offenders."

Earlier in the term, among other things, the court allowed state governments to be sued for violating the Family and Medical Leave Act and ruled that noncitizens facing deportation for illegal activity could be held without due process.

All of these decisions, plus the two California cases, "had one common characteristic," Chemerinsky wrote. "Justice Sandra Day O’Connor was in the majority. Indeed, she was the only justice in the majority in every one of these cases."

At the end of the last term, in June, there was speculation that O’Connor might retire in order to give President Bush an opportunity to name someone to the court. She did not, and given the prospect of a major partisan battle if a vacancy were to occur in 2004, she is probably going to remain in place at least this term and next.

And why not? No one else in America has more authority in more areas of domestic policy than she does. And no one calculates her use of power more carefully.

David Broder can be reached at The Washington Post Writers Group, 1150 15th St. NW, Washington, DC 20071-9200 or at davidbroder@washpost.com.