THE REPLACEMENT of Supreme Court Justice Sandra Day O'Connor with Judge John G. Roberts Jr. could have a significant impact on the law, and on the country. How much impact is hard to know: While many people presume to know Judge Roberts's views, not much can be learned from the public record. Nominees are traditionally -- and rightly -- reticent about answering questions that could commit them on specific cases. All of which leaves senators in something of a bind. The stakes are high, and information will be tight.

Public discussion of the nomination has unsurprisingly focused so far on abortion rights. But abortion may not be the area where Judge Roberts will have the biggest effect -- at least not initially. He cannot provide the decisive vote for overturning Roe v. Wade, which is currently protected by a 6-to-3 majority, even if he favors doing so. What he could do is move the court toward tolerating a range of restrictions on abortion, such as the "partial-birth" procedure, that it currently does not permit. That would mean a unwarranted erosion of women's privacy rights.

Another area where Judge Roberts could have a big impact is on the court's approach to federalism -- that is, the balance of power between the national government and the states. The court seems less poised than it did only a few years ago to head in a radical direction in this area. One of Judge Roberts's opinions, however, raises worrisome questions about whether he would be willing to go too far in reining in federal power. We would hope he could instead provide a strong center that is now missing from the court on these questions.

Two other areas where Judge Roberts could shift the court are campaign finance regulation and affirmative action in education. In both areas, Justice O'Connor provided the decisive fifth vote for upholding challenged policies: the McCain-Feingold campaign finance law and race-conscious admissions at the University of Michigan. Judge Roberts's elevation raises the possibility of the court's taking a different course or even reconsidering those holdings, both of which we consider correct. It is particularly important that the court give Congress constitutional leeway to adopt effective campaign finance rules.

More generally, Justice O'Connor has been a key vote for expansions of judicial power in many areas -- some justified, some unwarranted in our view. She supported, for example, the court's assertion of jurisdiction over the Guantanamo Bay detentions, as well as its management of race-conscious redistricting. Judge Roberts would have to be only mildly more restrained in his attitude toward judicial authority to reduce the court's ever-growing profile. That could affect future judicial checks on the Bush administration's assertions of power in the war on terrorism, among other issues.

Teasing out what sort of justice someone will be is never easy -- particularly when that someone has been as careful in his public utterances as Judge Roberts. It will probably prove impossible to gauge how he will vote in specific case areas. It should, however, be possible to have a serious discussion with him about the general principles of constitutional decision making that underlie all of these questions. Judge Roberts has said he does not hew to a single all-encompassing theory of constitutional rulings -- an encouraging sign in and of itself. But what are his methods for interpreting the Constitution? How much deference should the court grant to its past decisions when they might have been in error, and under what circumstances should it reverse course and overturn major decisions? It should be possible, without demanding that the nominee compromise his independence as a justice, to learn a lot more about him.