Washington ? John G. Roberts Jr., a conservative protege of the late William H. Rehnquist, succeeded him Thursday and became the nation’s youngest chief justice in two centuries, winning support from more than three-fourths of the Senate after promising he would be no ideologue.

Roberts, at 50, becomes the 17th chief justice, presiding over a Supreme Court that seems as divided as the nation over abortion and other tumultuous social issues. The court opens a new term on Monday.

“The Senate has confirmed a man with an astute mind and kind heart,” President Bush said just before Roberts was sworn in by acting Chief Justice John Paul Stevens. “All Americans can be confident that the 17th chief justice of the United States will be prudent in exercising judicial power, firm in defending judicial independence and above all a faithful guardian of the Constitution.”

Bush is expected to make his second Supreme Court nomination within days, one that conservatives hope will move the court to the right. Replacing Rehnquist with Roberts keeps the court’s current balance, but replacing the moderate Justice Sandra Day O’Connor with a conservative could tilt it rightward.

Roberts called the Senate’s 78-22 bipartisan vote for him “confirmation of what is for me a bedrock principle, that judging is different from politics.” All of the Senate’s 55 Republicans, independent James Jeffords of Vermont and half of the 44 Democrats supported him.

Supreme Court Justice John Paul Stevens swears in John Roberts Thursday as the chief justice of the United States.

He said he would try to “pass on to my children’s generation a charter of self-government as strong and as vibrant as the one that Chief Justice Rehnquist passed on to us.”

“What Daniel Webster termed the miracle of our Constitution is not something that happens in every generation, but every generation in its turn must accept the responsibility of supporting and defending the Constitution and bearing true faith and allegiance to it,” Roberts said.

A crowd including seven of the eight sitting justices, Roberts’ parents, Rosemary and John Sr., children John and Josephine, Senate supporters and White House well-wishers erupted stood and applauded as Roberts kissed his wife and shook Stevens’ hand. The audience also included Attorney General Alberto Gonzales and White House counsel Harriet Miers, both of whom have been mentioned as candidates for O’Connor’s seat.

Roberts took a separate judicial oath during a private White House ceremony attended by the other justices. A formal Supreme court ceremony was scheduled for Monday, before the opening of the term.

O’Connor, a moderate voice on the Supreme Court and one of only two women, is leaving after 24 years. It is the first time in 34 years that a president has had simultaneous high court openings.

In this photo provided by the White House, President Bush signs the commission appointing John Roberts, right, as the 17th chief justice of the United States before swearing-in ceremonies.

The president originally named Roberts to succeed O’Connor in July. Rehnquist’s death led to the switch to Roberts for the chief justice on Sept. 6. O’Connor remains on the court until the president selects a replacement and that person is confirmed by the Republican-controlled Senate.

The only justice not at the White House was Antonin Scalia. He had a previous engagement that could not be broken, a court spokeswoman said. According the Federalist Society Web site, he was leading a two-day seminar on the separation of powers in Avon, Colo.

Not since John Marshall, confirmed in 1801 at 45, has there been a younger chief. Roberts is the first new Supreme Court justice since 1994.

Before becoming a federal appeals court judge, he was one of the nation’s best appellate lawyers, arguing 39 cases – many in front of the same eight justices he will now lead as chief. He won 25 of those cases.

Under Roberts, the court will tackle such issues as assisted suicide, campaign finance law and abortion this year, with questions about religion, same-sex marriage, the government’s war on terrorism and human cloning looming in the future.

Said Senate Majority Leader Bill Frist, R-Tenn: “For many years to come, long after many of us have left public service, the Roberts court will be deliberating on some of the most difficult and fundamental questions of U.S. law.”

Twenty-two Democrats opposed Roberts, saying he could turn out to be as conservative as Scalia and Clarence Thomas, the Supreme Court anchors on the right.

“At the end of the day, I have too many unanswered questions about the nominee to justify confirming him to this lifetime seat,” said Democratic leader Harry Reid of Nevada.

Anti-abortion and abortion rights activists both have their hopes pinned on Roberts, a former government lawyer in the Reagan and first Bush administrations. While Roberts is solidly conservative and his wife, Jane, volunteers for Feminists for Life, both sides were eager to see how he would vote on abortion cases.

Roberts told senators during his confirmation hearings that past Supreme Court rulings carry weight, including the Roe v. Wade decision that legalized abortion in 1973. He also said he agreed with the 1965 Supreme Court ruling in Griswold v. Connecticut that established the right of privacy in the sale and use of contraceptives.

But he tempered that by saying Supreme Court justices can overturn rulings.

“If the Constitution says that the little guy should win, then the little guy’s going to win in the court before me,” Roberts told senators. “But if the Constitution says that the big guy should win, well then the big guy’s going to win because my obligation is to the Constitution.”

Over and over, he has assured lawmakers his rulings would be guided by his understanding of the facts of cases, the law and the Constitution, not by his personal views. “My faith and my religious beliefs do not play a role,” said Roberts, who is Catholic.