President Bush tonight announced his nomination of John G. Roberts Jr., a federal appeals court judge, to fill the first Supreme Court vacancy of his presidency.

In a nationally televised ceremony in the East Room of the White House, Bush hailed Roberts, 50, as a man of "superb credentials and the highest integrity."

Bush called on the Senate to promptly confirm Roberts, a former law clerk to William H. Rehnquist when he was an associate justice on the court, who the president said enjoys bipartisan support. But two leading Democrats on the Senate Judiciary Committee, which initially takes up the nomination, said they would insist on thoroughly exploring Roberts's views on a wide range of issues.

Bush prefaced his introduction of Roberts by noting that the appointment of a Supreme Court justice is one of a president's "most consequential decisions," since Supreme Court rulings "affect the life of every American."

He said Roberts, a conservative former White House and Justice Department official in past Republican administrations who currently serves on the U.S. Court of Appeals for the D.C. Circuit, "has devoted his entire professional life to the cause of justice and is widely admired for his intellect, his sound judgment and personal decency." Bush said he had been "deeply impressed" by Roberts in his search for a successor to Justice Sandra Day O'Connor, who is retiring.

"He's a man of extraordinary accomplishment and ability," Bush said. "He has a good heart. He has the qualities Americans expect in a judge: experience, wisdom, fairness and civility."

Roberts "will strictly apply the Constitution and laws, not legislate from the bench," Bush said.

In brief remarks following Bush's introduction, Roberts said it was "both an honor and very humbling to be nominated to serve on the Supreme Court." He said that in arguing past cases before the Supreme Court as an attorney, he developed a deep appreciation for its role in American democracy and "always got a lump in my throat" as he walked up the building's steps.

Neither Bush nor Roberts took questions from reporters after the announcement. Instead, they turned and walked down a hallway with Roberts's wife, Jane, who had stood to the side looking on as the two men spoke.

Word of the nomination leaked out in advance of the 9 p.m. ceremony.

Roberts, long considered one of the Republicans' legal heavyweights, was appointed to the D.C. federal appeals court in 2003 by Bush. He was also nominated to the same court by Bush's father, president George H.W. Bush, but never received a Senate vote.

Roberts practiced law at the Washington law firm, Hogan & Hartson, from 1986 to 1989 and from 1993 to 2003.

Between those periods, he was the principal deputy solicitor general in the George H.W. Bush administration, helping to formulate the administration's position in Supreme Court cases. He has argued more than three dozen cases before the Supreme Court.

During the Reagan administration, Roberts served for a time as an aide to Attorney General William French Smith. He also worked for White House Counsel Fred Fielding, from 1982 to 1986.

He attended Harvard College and the Harvard Law School. Early in his career, he clerked for Justice (now Chief Justice) William H. Rehnquist on the Supreme Court.

Roberts has not been considered a "movement conservative," and in the past, some on the Republican Party's right have doubted his commitment to their cause.

As one of the government's top lawyers before the Supreme Court, he argued in 1990 in favor of a government regulation that banned abortion-related counseling by federally-funded family planning programs. A line in his brief noted the first Bush administration's belief that Roe v. Wade, the 1973 Supreme Court decision that legalized abortion, should be overruled. The brief said the court's conclusion "that there is a fundamental right to an abortion . . . finds no support in the text, structure or history of the Constitution."

In Roberts's confirmation hearings in 2003, he and Sen. Orrin Hatch (R-Utah) noted repeatedly that in taking this and other positions, Roberts was doing his job representing the administration and not voicing his personal views.

Roberts, a native of Buffalo, N.Y., told senators at that time that "Roe v. Wade is the settled law of the land." He said, "There is nothing in my personal views that would prevent me from fully and faithfully applying that precedent."

As a federal judge, however, his record is less than two years old, and opinions on which to judge him are therefore limited.

In one case before the appeals court last year, Roberts attracted some attention as the author of Hedgepeth v. WMATA, a highly publicized case involving the arrest of 12-year-old girl for eating French fries in a Metrorail station. Roberts, ruling for the court, noted that "no one is very happy about the events that led to this litigation." He added, however, that "the question before us is not whether these policies were a bad idea, but whether they violated the Fourth and Fifth Amendments to the Constitution." The three-judge panel ruled unanimously that the policies were constitutional.

Initial reaction to Roberts from Republican senators was laudatory, while Democrats praised his abilities but indicated they were withholding judgment until the confirmation hearings.

In a press conference following Bush's announcement, Sen. Patrick Leahy (D-Vt.), the ranking Democrat on the Judiciary Committee, said that "we need to consider this nomination as thoroughly and carefully as the American people deserve." He said the confirmation process is "going to take time," adding, "No one is entitled to a free pass to a lifetime appointment to the Supreme Court."

Sen. Charles Schumer of New York, another Democrat on the committee, said, "There's no question that Judge Roberts has outstanding legal credentials and an appropriate legal temperament and demeanor. But his actual judicial record is limited to only two years on the D.C. Circuit Court. For the rest of his career, he has been arguing cases an as able lawyer for others, leaving many of his personal views unknown."

Schumer said Roberts must "answer a wide range of questions openly, honestly and fully in the coming months." He said he had voted against Roberts for the D.C. Court of Appeals "because he didn't answer questions fully and openly when he appeared before the committee."

Sen. John R. Warner (R-Va.) hailed Roberts as "a good choice." Sen. Joseph I. Lieberman (D-Conn.) had previously identified him as a choice who would not trigger a Democratic filibuster. Both senators were among 14 lawmakers -- seven Republicans and seven Democrats -- who pushed through a compromise that averted a Senate showdown over the blockage of Bush judicial nominees through filibusters.

Sen. John Cornyn (R-Tex.), a former judge whose name had been mentioned as a potential Supreme Court nominee, called Roberts "an exceptional judge, brilliant legal mind and a man of outstanding character."

Sen. Harry M. Reid (D-Nev.), the Senate minority leader, said, "The president has chosen someone with suitable legal credentials, but that is not the end of our inquiry." He said the Senate must determine whether Roberts "has a demonstrated commitment to the core American values of freedom, equality and fairness."

The nomination came after a day of intense speculation on the identity of Bush's choice to be the first new Supreme Court justice in 11 years. The White House announced earlier in the day that Bush had made up his mind on a replacement for O'Connor, 75, who declared on July 1 that she would retire upon the confirmation of her successor by the U.S. Senate.

The nomination carries high stakes politically because justices are lifetime appointees whose decisions can affect major national issues for decades.

With Rehnquist determined to stay on as chief justice despite a battle with thyroid cancer, the retirement of O'Connor gave Bush his first chance to leave his mark on the high court. O'Connor, an appointee of president Ronald Reagan, was the first woman to be named to the court, and Bush lately had come under pressure -- from his wife, among others -- to appoint another woman to replace her.

One potential replacement whose name was bruited about today was Edith Brown Clement, a judge on the U.S. Court of Appeals for the 5th Circuit, based in New Orleans. But as the day wore on, White House sources were cautioning against such speculation.

Others whose names remained in the mix before Bush's decision were Attorney General Alberto R. Gonzales, former deputy attorney general Larry Thompson and federal appeals court judges J. Michael Luttig and Emilio Garza.

Reid, the Senate minority leader, said before Bush's announcement that the Senate confirmation process could go relatively quickly if the president selected "a noncontroversial candidate." In that case, Reid said, confirmation could be completed by Oct. 1, in time for the Supreme Court's next session.

The Senate's top Democrat also noted that all the talk about a Supreme Court nominee had pushed aside a growing controversy over the role of Karl Rove, the White House deputy chief of staff and Bush's top political strategist, in the leaking of a covert CIA agent's identity.

Sen. Mitch McConnell (R-Ky.), the majority whip, told reporters that the Senate owes the public "a respectful process during the committee hearings and the floor consideration" of Bush's choice. He said Bush went through "unprecedented" consultations before making his pick and that "a clear majority of the Senate was consulted during this process."