“He’s very concerned about the activism of the court in recent terms,” said Senator Patrick J. Leahy of Vermont, chairman of the Judiciary Committee, ticking off a series of cases that angered liberals, most notably allowing corporations to spend freely in election campaigns. “He wants to make sure he puts somebody on there who is not going to take radical steps like that.”

But the chief justice’s defenders said his rulings have simply upheld a textual analysis of the Constitution and they cast Mr. Obama’s focus on the court as opportunistic and ideological. “The president is willing to attack the Supreme Court in a calculated political manner,” said Shannen W. Coffin, a friend of Chief Justice Roberts’s and a former counsel to Vice President Dick Cheney.

Either way, the search for a replacement for the retiring Justice John Paul Stevens is centered on finding a justice who will not just replicate his liberal votes but also bring intellectual heft and powers of persuasion to the court to win the swing vote of Justice Anthony M. Kennedy, according to people close to the search who insisted on anonymity to discuss it. While activists on the left often say they want a liberal Antonin Scalia, the fiery conservative justice, Mr. Obama is looking for a liberal John Roberts, who can forge a five-vote majority rather than write satisfying but ultimately meaningless dissents.