One key player on the Democratic side of the debate on Supreme Court nominees won't cast a confirmation vote yet may be nearly as important as the senators who do: Professor Cass Sunstein of the University of Chicago. Those who watched closely during the Judiciary Committee's hearing on John Roberts will remember the name, if not the legal theories, of Mr. Sunstein.

Like the new chief justice, Mr. Sunstein graduated from Harvard College, where he played varsity squash, and Harvard Law School. Both went on to clerk at the Supreme Court; Mr. Roberts for Justice Rehnquist, and Mr. Sunstein, who got out of law school a year ahead of Mr. Roberts, for Justice Thurgood Marshall.

Mr. Sunstein was cited by both Republicans and Democrats during the Roberts hearing last month. The most pointed question came from Senator Hatch, a Republican, who asked Judge Roberts whether he had read Mr. Sunstein's latest: "Radicals in Robes: Why Extreme Right Wing Courts are Wrong for America."

"I didn't have a chance to read Professor Sunstein's book," Judge Roberts said. "He writes a different one every week. It's hard to keep up with him."

The question was a significant one. Four months after Mr. Bush was sworn in to serve his first term, Mr. Sunstein and a handful of other legal theorists schooled 40 Democratic senators during a retreat in Farmington, Pa. The topic of the session was the new president's potential impact on the courts. One month later, Senator Schumer, the Democrat of New York who sits on the judiciary committee, published a now-famous New York Times article in which he argued that ideology should play a more explicit part in the screening of judicial nominees. Though Mr. Schumer's aides would not confirm whether the two men collaborate, the piece bore the imprint of Mr. Sunstein's ideas.

Mr. Sunstein, 51, is reticent about discussing the extent to which he advises Democratic senators. "Professors shouldn't brag about talking to U.S. senators," he wrote to me in an e-mail last week. "I know that U.S. senators talk to a ton of people, including academics, and I guess I think it's up to them to say whom they've called." But it is worth noting that he participated in a panel discussion on the court co-sponsored by the American Enterprise Institute and the Brookings Institution in Washington, D.C., the day that Mr. Hatch asked Judge Roberts where he fit into Mr. Sunstein's list of today's competing judicial philosophies.

Mr. Sunstein's panel appearance also took place on the same day that Mr. Schumer surprised a number court watchers and political observers by saying toward the end of a long hearing day that he was "pleasantly surprised" by Mr. Roberts' explanation of privacy rights from earlier that day. Readers of Mr. Sunstein may have been the only people on Capitol Hill who were not surprised by Mr. Schumer's repeated praise for a conservative court nominee: In Mr. Sunstein's view, Judge Roberts is extremely cautious, "a minimalist," in his approach to the Constitution - the least dangerous of the three types of judges that he identifies in "Radicals in Robes."

The other types of judge, according to Mr. Sunstein, are fundamentalists, or those who take the Constitution at face value, and perfectionists, those who use the Constitution as a lever for effecting ideals. Interestingly, Mr. Sunstein, a contributing editor at the liberal magazine The New Republic, has described the court's Roe v. Wade decision that overturned state laws limiting abortion as the work of a perfectionist court. The fact that a chief adviser to the Democrats has described this case as flawed is another reason many wonder why Mr. Bush has favored stealth in picking nominees.

Is the president's latest Supreme Court nominee, Harriet Miers, a fundamentalist? On religion, the White House has come under criticism even from religious Republicans for trying to convince prominent evangelicals that her apparent Biblical fundamentalism is reason for supporting her nomination. A better strategy might be trying to convince Mr. Sunstein that Ms. Miers is not a fundamentalist when it comes to the Constitution. Democrats - if not conservatives - will be listening.

The fact that Mr. Sunstein does not seem all that concerned about Ms. Miers's religion suggests that evangelical Christians who viewed this as a reason to support her might have rushed to judgment. "I don't think her religion should be an issue," Mr. Sunstein wrote in another e-mail last week. "Plenty of religious fundamentalists don't believe in constitutional fundamentalism." It's a distinction that plenty of Democratic senators will be trying to make during Ms. Miers's confirmation hearings.

Mr. McGuire is a Washington correspondent of The New York Sun.