Here is what liberals and conservatives can agree on about John Roberts, President Bush's first Supreme Court nominee: He is perhaps the most impressive Supreme Court advocate of his generation, extremely intelligent, thoughtful and able — a lawyer's lawyer. In a reasonable world, that should be enough to assure his confirmation with bipartisan enthusiasm. Unfortunately, Washington politics is anything but reasonable.

Judge Roberts takes pride in representing both sides of the political spectrum. He delighted environmental groups by convincing the Supreme Court that a freeze on development in an unspoiled part of Lake Tahoe didn't violate the private property rights of the affected landowners. He has argued for and against the constitutionality of affirmative action. For Roberts, the ability to "argue a case round or argue it flat," as the lawyers say, is a point of pride.

As both an appellate lawyer and an appellate judge, he earned the reputation of a legal craftsman who didn't come to cases with preconceived grand theories but took positions based on the arguments and legal materials in each case.

Roberts is, by all accounts, a very nice man: funny, humble and decent. He treats judges and litigants with a Jimmy Stewart-like courtesy. He sends notes to associates whose children are sick. His winning personality has raised the hopes of conservatives who understand that the most influential justices are those who work well with their colleagues.

But here is where conservatives and liberals may part company about Roberts: Conservatives hope he will be a William Brennan of the right, using his intelligence and charm on behalf of his deeply conservative views to move the court far to the right of where it was under the moderate influence of Justice Sandra Day O'Connor; liberals fear that conservatives are correct.

Liberals worry that a Justice Roberts might take a narrow view of Congress' power to regulate the economy that would impose severe limitations on the regulatory state. And they fear that he would vote to overturn Roe v. Wade, because in 1990, when he was a deputy solicitor general, he signed a brief in an abortion-financing case that included a footnote calling for Roe to be overturned.

How can the Senate cast light on the question of whether Roberts is a conservative ideologue with an agenda to transform the law or a conservative incrementalist who may surprise liberals and conservatives alike with his independence?

To begin with, senators should forget about the government briefs Roberts signed about Roe v. Wade, school prayer and other hot-button issues. It's clearly not fair to hold him accountable for defending the George H.W. Bush administration's official positions. After all, that was, at the time, his job.

Instead, the Senate should explore Roberts' judicial philosophy and temperament. He has been on the appellate court for only two years, however, so clues in his judicial record are necessarily sparse.

But based on his record throughout his career, he does not appear to be a rigid Constitutional "originalist" in the tradition of Justices Antonin Scalia and Clarence Thomas. These men believe that the Constitution should be strictly interpreted in light of its original understanding; they are willing (to different degrees) to overturn years of Supreme Court precedents in the name of constitutional fidelity.

Having spent decades arguing before courts rather than sitting on them, Roberts has never embraced one grand legal theory to the exclusion of all others. On the contrary, he has been trained to cast a wide net in order to reach a convincing result. Inflexible originalism is a theory embraced by academics and crusaders, not practicing lawyers who must persuade judges of different stripes.

At the same time, Roberts is not a former legislator, and therefore he is not likely to be as willing to split every difference between liberals and conservatives. As an appellate lawyer forced to apply legal precedents, he was trained to believe that judges should provide clear answers to legal questions rather than keeping the country guessing.

So, are conservatives right to hope, and liberals right to fear, that as a justice, he would vote to overturn many of the 5-4 cases where O'Connor sided with her more liberal colleagues? The best way for the senators to find an answer to this question is to explore Roberts' view of precedents, which the lawyers call stare decisis, or "let the decision stand."

In the confirmation hearings for his appellate judgeship, Roberts said he was bound to apply the Supreme Court's precedents. That was a good answer at the time, but it is no longer terribly relevant: as a Supreme Court justice, he would be free to overturn the court's earlier rulings.

The truth is that Roberts probably doesn't have a well-thought-out theory of stare decisis. As an appellate lawyer and judge, he had no need or occasion to develop one.

In fact, very few Supreme Court justices have developed a theory of stare decisis that is entirely satisfying. At one extreme there is Thomas, who, according to his colleague Scalia, is willing to overturn any precedent he thinks is inconsistent with the original understanding of the Constitution. At another extreme have been justices like John Marshall Harlan, who, in the name of judicial continuity, are very reluctant to overturn precedents, even those with which they disagree.

Perhaps one clue to Roberts' leanings on the force of precedents can be found in the outlook of one of his judicial heroes, Henry Friendly, an appellate judge for whom he became a clerk in 1979. Friendly was famously cautious, a man devoted to incremental rather than radical legal change. It might be illuminating for the senators to ask Roberts what he admired about Friendly, and why.

Another potentially fruitful line of questioning might center on Roberts' views about the scope of Congress' power to regulate the environment and the economy.

As an appellate judge, his record on this crucial issue has been indistinct. In one case, he took an expansive view of Congress' power to condition the receipt of public money on an agency's promise not to discriminate. In another, he took a much more restrictive view of Congress' power to regulate the environment.

Some of his comments suggest that he thinks the court is correct to strike down federal laws on rare occasions, but that he may be unlikely to try to resurrect what some conservatives call "the Constitution in exile," overturning decades of precedents and dismantling the regulatory state root and branch.

While it is appropriate for senators to ask Roberts about specific cases, they might get him to reveal more of himself if they asked him about his vision of the role of the courts in democracy.

When I interviewed him three years ago, I was impressed with his reverence for the law as something distinct from politics, his belief that courts should operate according to independent ideals of professionalism and neutrality, and, most of all, his apparent lack of anger.

If his confirmation hearings confirm this impression, Roberts may prove to be not only a great justice, but one whom principled liberals can embrace with gratitude and relief.

Jeffrey Rosen is a law professor at George Washington University and the legal affairs editor of New Republic magazine.