PRESIDENT BUSH lost no time in naming a successor for Chief Justice William H. Rehnquist. If confirmed, Judge John G. Roberts Jr. will bring the same virtues to the job of heading the country's judiciary as he would have brought to the job of associate justice. A highly regarded former appellate lawyer and current appeals court judge, he is a modest, smart lawyer who, unlike some of the president's judicial picks, is generally well regarded across party lines. Mr. Bush deserves credit for acting with dispatch. The Senate should do so as well and, giving a full airing to the issues, should make a point of voting on the nomination before the court reconvenes in October.

In most respects, the change in position ought to signify little in terms of how senators regard Judge Roberts. The difference between the chief justice and the other members of the court, after all, is largely administrative, not substantive; all justices have only one vote. Judge Roberts is, to be sure, on the young side for the job of chief. At 50, he will have, if confirmed, the odd distinction of being the youngest member of the court he heads. On the other hand, his easy manner and collegiality ought to serve him well.

Yet his nomination as chief justice nonetheless raises the stakes in the discussion of what Judge Roberts actually believes -- a subject about which a great deal more has been said than is really known. Liberal groups that feared what a Justice Roberts might do can only fear a Chief Justice Roberts all the more. The hearings will provide an excellent opportunity for the Senate Judiciary Committee to explore -- and for Judge Roberts to address -- a raft of important questions: What is his attitude toward what is called stare decisis, the doctrine of generally letting past cases stand even if wrongly decided? How does he regard the balance of power between the federal government and the states, an ongoing debate in which a new justice could play a dramatic role, either constructive or unfortunate. What does he believe about privacy rights?

Documents from Judge Roberts's service in the Reagan administration -- in the Justice Department and in the White House counsel's office -- have raised fears that he might harbor reactionary views on a range of civil rights questions. Some of this is overblown; opponents have tried to make controversy out of reasonable positions Mr. Roberts took against policies few still advocate today. On the other hand, Judge Roberts opposed changes to the Voting Rights Act that had broad bipartisan support, as well as affirmative action programs and the proposed Equal Rights Amendment. He seemed deeply suspicious of federal review of state convictions. He seems to have objected to decades of Supreme Court rulings restricting the role of religion in public life. Judge Roberts was quite young when these memorandums were drafted, and his views may have evolved as he matured. Which part of this record as a young lawyer does he stand behind now, and which part does he not? All of these questions take on a greater sense of importance -- at least symbolically -- with Judge Roberts now nominated to be chief.

In one critical respect, however, moving Judge Roberts to the chief's slot may actually reduce his impact on the court's ideological balance. Chief Justice Rehnquist, unlike Justice Sandra Day O'Connor, was not often a swing vote on the court. In replacing him, Judge Roberts will be very unlikely to move the court substantially to the right. He would not have to prove all that surprising to move it to the left on certain issues.

This fact, in turn, puts a heavy premium on Mr. Bush's choice regarding a new replacement for Justice O'Connor -- whose decision to stay on the court until a nominee garners confirmation is proving invaluable at a delicate transitional moment. Before nominating Judge Roberts, Mr. Bush consulted widely and emerged with a candidate Senate Democrats have -- by and large, anyway -- treated with respect. As he once again considers a nominee to replace Justice O'Connor, Mr. Bush would do a public service by consulting widely and meaningfully.