WASHINGTON — President Bush on Monday nominated a federal appeals court judge with unquestioned conservative credentials to fill the vacancy on the Supreme Court, satisfying his supporters on the political right but inviting a knockdown battle with Democrats.

Bush's choice, Samuel A. Alito Jr., is a 15-year veteran of the Philadelphia-based U.S. Court of Appeals for the 3rd Circuit, where he has participated in 3,500 cases and written about 300 opinions. He was named to that job by Bush's father.

Bush moved quickly to replace a failed nomination, that of White House counsel Harriet Miers, who withdrew as a Supreme Court pick on Thursday after she ran into harsh opposition from conservatives. Announcing his decision at the White House with Alito by his side, the president called Alito "one of the most accomplished and respected judges in America."

If confirmed by the Senate, the 55-year-old Alito would replace retiring Justice Sandra Day O'Connor, who has been a centrist and the swing vote in many closely decided cases.

Conservatives welcomed the choice. "President Bush has hit a home run," said Sen. Trent Lott, R-Miss.

But liberal interest groups and some Democrats voiced immediate concerns, particularly because the choice may prove pivotal on the high court's future decisions on issues from abortion to civil rights and civil liberties.

"I would have hoped the president would have filled Sandra Day O'Connor's seat with a person in line with her philosophy," said Sen. Mary Landrieu, D-La. "This is a needlessly provocative nomination," said Sen. Patrick Leahy, D-Vt., the senior Democrat on the Judiciary Committee.

Landrieu and 13 other senators from both parties joined in a truce earlier this year to head off a threatened filibuster over Bush's court nominations. They declared that they would not support such roadblock tactics unless "extraordinary circumstances" required it. For those who agreed to the deal, the question now is whether the Alito nomination meets that test.

A Republican member of the so-called "Gang of 14," Sen. Lindsey Graham of South Carolina, signaled that he would not support a filibuster. If Democrats try, it "will create a Middle East style of politics in the Senate," he said. But, asked about the possibility of a filibuster, Senate Minority Leader Harry Reid, D-Nev., said: "I wouldn't take it off the table."

Bush seemed to acknowledge the delicacy of the situation when he introduced his nominee. He said Alito "has a deep understanding of the proper role of judges in our society. He understands that judges are to interpret the laws, not to impose their preferences or priorities."