A Supreme battle? Samuel Alito's nomination to the Supreme Court sets the stage for an ideological fight in the Senate.

Before this summer, there had not been a vacancy on the 9-member Supreme Court for a decade. For much of that time, liberals and conservatives were preparing for a fight that both sides see as affecting the Court's direction for decades to come.

That's why the stakes are so high in the battle over Judge Samuel A. Alito Jr., 55, whom President Bush has nominated to fill the seat of Justice Sandra Day O'Connor.

While vacancies on the Court have long been expected, the past few months have seen a flurry of activity. In July, O'Connor announced her intention to retire, and Bush nominated John G. Roberts Jr. to replace her. But before his Senate confirmation hearings began, Chief Justice William Rehnquist died of thyroid cancer and Bush shifted his nomination of Roberts to fill the Chief Justice's seat instead.

SOLIDLY CONSERVATIVE

Roberts was confirmed by a 78-22 Senate vote in September, after very civil hearings. In early October, Bush selected White House Counsel Harriet E. Miers to fill O'Connor's seat, but her nomination proved increasingly controversial, and she withdrew three weeks later.

Alito, her replacement, has been a judge on the U.S. Court of Appeals for the Third Circuit in Philadelphia for 15 years. His decisions, legal scholars say, have been methodical, respectful of precedent, and solidly conservative. O'Connor has been the swing vote on many issues, so replacing her with a staunch conservative could significantly affect the balance of the Court. Not surprisingly, liberal groups have called Alito a threat to abortion rights, civil liberties, and gun control, while conservative groups have given him glowing endorsements.

Debate over the criteria the Senate should use in considering nominees is as old as the Constitution itself, which says only that judges will be appointed by the President "with the advice and consent of the Senate."

SHOULD IDEOLOGY COUNT?

Should education, experience, and integrity be the determining factors? Or should ideology, a nominee's political leanings and predictable stands on the judicial disputes of the day also play a role? Alito's 15-year paper trail of judicial opinions will give opponents ammunition to use against him.

The son of an Italian immigrant, Alito went to Princeton and Yale Law School. He has worked in the Justice Department and as a U.S. Attorney. Senate hearings on Alito's nomination are set for January.

Todd S. Purdum is a Washington correspondent for The New York Times; additional reporting by Neil A. Lewis and Scott Shane of The Times.