WASHINGTON — President Bush named federal appeals court judge John G. Roberts Jr. to a seat on the Supreme Court Tuesday, delighting Republicans while unsettling some Democrats with the selection of a young jurist with impeccable conservative credentials.

"John Roberts has devoted his entire professional life to the cause of justice," Bush said in a prime-time announcement at the White House, "and is widely admired for his intellect, his sound judgment and his personal decency."

If confirmed by the Republican-controlled Senate, the 50-year-old Roberts would succeed retiring Justice Sandra Day O'Connor, who has long been a swing vote on a court divided narrowly on issues such as abortion, affirmative action, states' rights and the death penalty.

Roberts stood at Bush's side as the president heaped praise on him, calling him "one of the finest legal minds" in the country.

The president said he had recently spoken with Senate leaders of both parties and said they "share my goal" of confirmation proceedings conducted with dignity and fairness.

In brief remarks, Roberts said it "is both an honor and very humbling to be nominated to serve on the Supreme Court." He said he has argued numerous cases before the high court during his career, adding, "I always got a lump in my throat whenever I walked up those marble steps to argue a case before the court, and I don't think it was just from the nerves."

The Harvard-educated Roberts learned of his selection in a lunchtime phone call from the president, according to administration officials. White House aides arranged for a prime time formal announcement as they sought the widest possible audience for a president making his first pick to the court — and the nation's first in more than a decade.

Initial reaction from Republicans was strongly in favor of Roberts. Sen. Jeff Sessions of Alabama called him a "fabulous nominee" and predicted that if confirmed, he would "bring a nonpolitical approach to judging."

Sen. Rick Santorum, R-Pa., a leading conservative, called him "brilliant.

Democratic reaction was more measured, but initially at least, offered no hint of a filibuster. "The president has chosen someone with suitable legal credentials, but that is not the end of our inquiry," said Senate Democratic leader Harry Reid of Nevada. Referring to planned hearings in the Senate Judiciary Committee, Reid said, "I will not prejudge this nomination. I look forward to learning more about Judge Roberts."

"Who knows about this guy?" said Sen. Tom Harkin, D-Iowa.

The abortion rights group NARAL Pro-Choice America immediately announced its opposition to Roberts.