WASHINGTON, Jan. 31 — Judge Samuel A. Alito Jr. was quietly sworn in Tuesday as the 110th justice of the Supreme Court, taking the oath of office less than two hours after a sharply divided Senate voted roughly along party lines to confirm him.
His confirmation, by a vote of 58 to 42, is expected to tilt the balance of the court to the right on matters like abortion, affirmative action and the death penalty, and partisans on each side said the outcome would echo through American politics for decades.
Judge Alito and his wife, Martha-Ann, watched the vote on television in the Roosevelt Room of the White House with President Bush and his wife, Laura, and aides who had worked on the nomination.
A cheer went up in the room as the vote count passed 50, and Judge Alito and his wife proceeded to the Supreme Court for a private swearing-in. His oath was administered by Chief Justice John G. Roberts Jr., who worked with Judge Alito as a lawyer in the Reagan administration, and the two men sat with other members of the Supreme Court at for the president's State of the Union address on Tuesday night.
For Mr. Bush, the confirmations of Justice Alito, 55, and Chief Justice Roberts, 51, mean that in four months he has named two members of the court who are young enough to influence its rulings for many years.
In a statement, Mr. Bush congratulated Justice Alito, the son of an Italian immigrant, saying his "appointment to the Supreme Court is the realization of the American dream." Mr. Bush is scheduled to commemorate Justice Alito's public swearing-in with a more elaborate ceremony on Wednesday at the White House.
Justice Alito succeeds Justice Sandra Day O'Connor, the first woman on the court and its swing vote on abortion rights and other social issues. After announcing her retirement last July, Justice O'Connor postponed her departure until a successor was confirmed. She is scheduled to return to her home state by Thursday to begin teaching a class on the Supreme Court at the University of Arizona in Tucson.
The vote on the nomination was unusually close and partisan. In the last 100 years of Supreme Court confirmations, only one vote was closer: the 52-to-48 decision to confirm Justice Clarence Thomas in 1991.
In this confirmation, 54 Republicans voted for Judge Alito, and just one, Senator Lincoln Chafee of Rhode Island, voted against him. Only four Democrats — Senators Ben Nelson of Nebraska, Kent Conrad of North Dakota, Tim Johnson of South Dakota and Robert C. Byrd of West Virginia — voted for confirmation, while the other 40 Democrats and the one independent voted against it.
The senators sat at their desks as the roll was called to reflect the solemnity of the occasion, and Mr. Byrd, 88, the oldest and longest-serving member of the Senate, made his way to his desk with canes in each hand to cast the last vote with a thumbs-up gesture.
Although only four Democrats crossed party lines, the vote reflected a deep divide within the party over how hard to fight Judge Alito's confirmation. An effort by a handful of Democrats to stage a filibuster over the objection of party leaders failed on Monday, when only 25 senators voted against closing debate.
The 42 senators who voted against confirmation would have been enough to block the nomination if they had voted against closing the debate. But many Democrats were unwilling to do so because it would have drawn charges of obstructionism from Republicans, who have threatened to change Senate rules to bar filibusters on judicial nominees.
Still, Democrats vowed to make an issue of Justice Alito and Chief Justice Roberts's decisions in elections this fall and beyond. "We will be watching our two newest justices," Senator Charles E. Schumer, a New York Democrat on the Judiciary Committee and the chairman of the Democratic Senatorial Campaign Committee, said in a speech before the vote.
"Make no mistake, we will make sure the American people understand the implications of these votes today," Mr. Schumer said. "Elections may have consequences, Mr. President, but votes like these also have consequences for future elections."
Officials of liberal groups, stung by the opposition of nearly half of the Democratic caucus to a filibuster of Judge Alito's nomination, criticized the party as giving in. "Today the Senate caved," said Wade Henderson, executive director of the Leadership Conference on Civil Rights.
The groups also pledged to make the Supreme Court their rallying cry in the midterm elections this fall. "Why did we lose?" said an e-mail message from Naral Pro-Choice America to its supporters. "One word sums it up: numbers."
The message continued, "We don't have the majority in the Senate."
Karen Pearl, interim president of Planned Parenthood and its political action fund, said many abortion-rights supporters had taken the Supreme Court for granted. "This nomination has moved the whole issue of reproductive health and freedoms much higher on voters' agenda, and I think they will remember this vote," Ms. Pearl said.
Conservatives, who have campaigned for decades against the court's decisions on abortion rights and the government's support for religion, celebrated Justice Alito's confirmation as a historic victory. The Family Research Council, a conservative Christian group, called the vote "a turning point for our nation."
"This is obviously a great day for us," said James C. Dobson, founder of the council and Focus on the Family. "We have not only been working toward this day, but praying for it for 20 years. There were times when it looked like we would never see a more conservative philosophy on the Supreme Court, and that makes today's development all the sweeter."
Paul M. Weyrich, chairman of the Free Congress Foundation and a conservative organizer, argued that Mr. Bush's pledge to nominate jurists like the conservative Justices Antonin Scalia and Clarence Thomas was the theme that held together his political coalition despite concerns about the war in Iraq, spending, immigration and other matters. "This is a reward, if you will, by Bush, in keeping his promise."
Both sides agree that liberals are likely to be on the defensive in future battles over the court. The youngest members of the court are all conservatives: Chief Justice Roberts, Justice Alito and Justice Thomas, 57. The oldest and most likely to leave the court next are liberals: Justice Ruth Bader Ginsburg, 72, and Justice John Paul Stevens, 85.