He added, "I believe that Democrats and Republicans alike will see the strong qualifications of this fine judge."
Mr. Bush's dramatic prime-time announcement ended more than two weeks of speculation set off by Justice O'Connor's surprise retirement announcement, and a day of frantic rumors in which first one then another candidate was reported to be the leading choice, amid hints from some Republicans that he might choose a woman to succeed the Supreme Court's first female justice.
Instead, word came shortly before 8 p.m. that Mr. Bush's choice was Judge Roberts, 50, a summa cum laude graduate of Harvard College, former managing editor of the Harvard Law Review and clerk to William H. Rehnquist, who was then an associate justice on the Supreme Court. Since 2003, Judge Roberts has served on the United States Court of Appeals for the District of Columbia Circuit, to which he was confirmed by unanimous consent of the Senate.
Mr. Bush has made no secret of his desire to impose a more conservative stamp on the Supreme Court, and he apparently named Mr. Roberts with confidence that he would help him do so.
Almost instantly, the conservative and liberal interest groups that have spent years preparing for a Supreme Court vacancy swung into action.
The conservative Progress for America called Judge Roberts a "terrific nominee," while Naral Pro-Choice America denounced him as an "unsuitable choice," and a "divisive nominee with a record of seeking to impose a political agenda on the courts."
But significantly, Senator Harry Reid of Nevada, the Democratic leader of the body that will determine Judge Roberts's fate, was much more subdued, hewing to the Democrats' stated strategy of demanding a thorough vetting of any nominee by describing Judge Roberts as "someone with suitable legal credentials," whose record must now be examined "to determine if he has a demonstrated commitment to the core American values of freedom, equality and fairness."
In his campaign for the presidency five years ago, Mr. Bush pledged to appoint judges like Justices Antonin Scalia and Clarence Thomas, staunch conservatives with well-established judicial philosophies.
While Judge Roberts has impeccable Republican credentials and a record of service in the Reagan and first Bush administrations dating to 1981, his paper trail of opinions is comparatively thin, and he is not seen as a "movement conservative."
Justice O'Connor had long ago emerged as the fulcrum of the current court, a pivotal vote on abortion, affirmative action, the death penalty and religion. Judge Roberts's detailed views on many of those issues are less known.
Abortion rights groups fault him for arguing, as deputy solicitor general for the first Bush administration in 1990, in favor of a government regulation banning abortion-related counseling in federally financed family planning programs.
He also helped write a brief then that restated the administration's opposition to Roe v. Wade, the 1973 Supreme Court decision that established the constitutional right to abortion, contending, "We continue to believe that Roe was wrongly decided and should be overruled."
But when pressed in his 2003 confirmation hearings for his own views, he said: "Roe v. Wade is the settled law of the land," and added, "There's nothing in my personal views that would prevent me from fully and faithfully applying that precedent."
Such comments have made Judge Roberts somewhat suspect in the eyes of some social conservatives. But he arouses nothing like the opposition that conservatives leveled at another potential nominee, Attorney General Alberto R. Gonzales, whose views on abortion are more uncertain.