On a Friday in October 1990, the Justice Department got some bad news from the clerk of the Supreme Court: The justices had disqualified a young assistant solicitor general scheduled to argue a highly technical bankruptcy case the next Monday.

For a replacement, Solicitor General Kenneth W. Starr tapped his 35-year-old principal deputy, John G. Roberts Jr. Roberts spent the weekend on the case, came to the court Monday morning and fielded questions from the justices for the government's allotted 10 minutes.

Then, in the afternoon, he went to the second-most-powerful court in town, the U.S. Court of Appeals for the D.C. Circuit, and argued another case -- a complicated financial dispute between the Environmental Protection Agency, which Roberts represented, and the water authority of Rochester, N.Y.

Roberts's side won both cases.

It is a story lawyers who practice before the Supreme Court tell with palpable awe. It illustrates why so many of them believe that Roberts is among the best in their profession -- and why they believe that his qualities would serve him well on the other side of the bench.

Although other justices in recent times have come from a career spent mostly in law practice, Roberts's nomination is the first in at least a century in which a former leader of the small, elite group of lawyers who regularly practice before the high court has been picked as a justice, according to Supreme Court historian Dennis J. Hutchinson of the University of Chicago.

To argue before the Supreme Court on only a couple of days' notice is rare. But to do so on the same day as appearing before another demanding federal appeals court is practically unheard of, lawyers say, a challenge that only the most confident and versatile of advocates would take on.

Carter G. Phillips, a leading Supreme Court lawyer, likened Roberts's performance to a race car driver's being thrown into the Indianapolis 500 in the morning, and winning, then earning a victory in a stock car race in the evening.

"The hardest part is the adrenaline," Phillips said. "You get this rush in the morning . . . and then it pours out at noon." To stand up before a second difficult tribunal while still riding this physical and mental roller coaster, he said, "would be very tough."

Although his experience as a Supreme Court lawyer has prepared Roberts to deal with the constitutional issues the court faces, it has offered relatively few opportunities to deal directly with ordinary citizens and mundane legal issues.

The vast majority -- 95 percent -- of his work has been in federal courts, not in the state courts where most U.S. legal proceedings take place, Roberts noted in a report to the Senate Judiciary Committee during his 2003 confirmation for the job he currently holds as a judge on the D.C. Circuit. Roberts added that he had worked on one jury trial in his career, and his involvement did not include appearing before the jury.

Also, the court under Chief Justice William H. Rehnquist is not known for a great deal of private discussion or debate about cases. Justices generally communicate with one another through short, typewritten notes. Roberts's persuasive talents may be less relevant in such a climate.

"There's so little interplay, that skill may not mean very much," Hutchinson said.

Roberts has argued before the Supreme Court 39 times -- 29 times as counsel for a party to a case and 10 times as a friend of the court. His side won 25 times.

His first opportunity came in January 1989, when he was 33; the justices asked him to represent a convicted felon facing more than $100,000 in additional civil fines. Years later, major corporations would hire Roberts to defend their interests before the justices, making him a millionaire.

Such corporate clients as Toyota, Peabody Coal and the health insurance industry were getting a particular kind of legal talent for the $650 to $700 per hour that competitors say an advocate of Roberts's stature could command.

Oral argument before the Supreme Court is unlike trial advocacy, in which the lawyer's goal is to get a jury to see the facts his way. In the Supreme Court and other appeals courts, the goal is to convince a set of judges that your interpretation of a disputed point of law is correct.

Strong emotional rhetoric may sometimes pay off in a trial, where the jury sits mute as the lawyer speaks. But at the Supreme Court, justices demand cool and thorough analysis, and they insist on it in a barrage of questions for which the lawyer must spend hours preparing.

Roberts's intellect and demeanor are well-suited to this kind of contest, former colleagues say.

"He was able to take complicated points, distill them to their essence and respond with an absolute minimum of verbiage, and make it seem that his argument is so obviously correct that you have no choice but to agree with him," said Jonathan S. Franklin, who worked with Roberts for 10 years when Roberts headed the appellate practice at Hogan & Hartson, a Washington law firm.

"He is comfortable up there," Phillips said. "He is confident enough to know that he could have a conversation with the justices."

When Roberts is conversing with them, the justices -- known as a "hot bench" for their frequent questioning -- spend an unusual amount of time listening.

In a 1992 argument, Roberts, representing the U.S. government, took the position that antiabortion demonstrators who had tried to block access to an abortion clinic were not liable for discrimination against women under federal law. He had only 10 minutes to speak, because the Bush administration was in the case only as a friend of the court on the demonstrators' side.

But, writes Supreme Court practitioner David C. Frederick in a book on appellate advocacy that is full of examples from Roberts's cases, he "had the luxury of making his points without any interruption from the court until he was about to sit down."

An oft-cited instance of Roberts's verbal adroitness occurred in a 1993 case. He was trying to convince the court that it was not cruel and unusual punishment for a prison to subject an inmate to exposure to secondhand cigarette smoke.

A justice asked if it would be permissible for the prison to subject inmates to asbestos exposure.

It would not, Roberts replied, because "we as a society do not treat exposure to asbestos as a matter of personal preference. When you go to a restaurant, they don't ask if you want the asbestos section or the non-asbestos section."

The courtroom erupted in laughter -- but Roberts may have been too clever by half. His side lost.

Still, the moment illustrated how difficult it has been for the justices to knock Roberts off balance.

In private practice, he would spend about five weeks preparing for each case, according to Frederick's book, "Supreme Court and Appellate Advocacy." During that process, Roberts would write down hundreds of potential questions and answers on notecards. He would also put his six or seven key points on notecards, then repeatedly shuffle the cards and practice making the arguments in different order.

Asked by Frederick whether clients always paid for that much preparation, Roberts noted that sometimes he did not charge for every hour spent on a case.

"But I long ago decided that telling the court, 'I don't know because the answer to that question wasn't in the budget,' was not what I wanted to say," he said.

Supreme Court cases are not decided solely or even mostly by oral argument. The justices have already waded through extensive briefs before each side argues in front of them.

But, Supreme Court practitioners say, an attorney's performance can sometimes tip the balance on close issues.

Roberts's first case before the court, U.S. v. Halper, involved a man who had been convicted of Medicaid fraud and sentenced to two years in prison and a $5,000 fine. The federal government also sued him for his offenses, and a court assessed him a $130,000 civil fine.

At issue was whether adding a civil penalty to a criminal one was double jeopardy, which is unconstitutional.

Roberts argued that it was and, by a vote of 9 to 0, the court agreed.

Eight years later, with five of the same justices serving -- but Roberts not arguing -- the court reversed itself, overruling the Halper decision 9 to 0.

"I've always suspected the quality of the advocacy may have accounted for the difference," said Franklin, Roberts's former partner.

A former colleague said when John G. Roberts Jr. argued before the Supreme Court, he could take on complex points and "distill them to their essence."