Irvin B. Nathan had his semi-retirement all figured out: a teaching post at a local law school — preferably American University’s, whose campus is a stroll from his Spring Valley home. After four decades of high-octane legal work, he thought, that would free up time for him and his wife to spend with their grandchildren in Upstate New York and at their vacation home in the Berkshires.

“I could take summers off, take it easy, exercise and lose some weight,” said Nathan, who turns 68 next month.

A phone call from an old friend changed that plan.

Today, Nathan is the District’s attorney general, running a $78 million office that for the first time in his distinguished career has him squarely in the rough-and-tumble of city politics.

During the first five months of his tenure, Nathan kept a relatively low profile, making news for his office’s role in fixing the police department’s botched breath-alcohol testing program and for an investigation into high gasoline prices. But on June 6, Nathan stepped front and center to unveil allegations that D.C. Council member Harry Thomas Jr. (D-Ward 5) diverted city money and charitable solicitations to groups he controlled, using the funds for a luxury sport-utility vehicle and golf trips. Thomas has vowed to fight the charges.

Never in city history has its top lawyer sued a sitting elected official. The probe, which had been launched by former attorney general Peter J. Nickles, has hastened an ongoing federal corruption investigation, and it has generated recognition and respect for Nathan in city political circles where he had been little known. And it has thrust him into a leading role dealing with the ethical challenges that have enveloped city government, including ones involving the man who appointed him, Mayor Vincent C. Gray (D).

“Being the attorney general in this city is a very, very difficult job, and recent events have sort of underscored that,” said Henry F. Schuelke III, a longtime friend of Nathan’s and a top white-collar defense attorney. “It’s a very difficult political environment. He is supremely well equipped to traverse this muddle, because he needs to do this like he needs a hole in his head.”

An unexpected opportunity

Nathan would not be in his position except for a particular set of circumstances: Last fall, he was out of a job. The House Democrats’ pounding in November’s elections meant he was likely to be replaced as the body’s top lawyer. Meanwhile, Gray, who had criticized Nickles for his close relationship with then-Mayor Adrian M. Fenty (D), was in search of a prestigious and fresh face. And Gray and Nathan had a mutual friend in Robert S. Bennett, the Hogan Lovells litigator best known for his representation of President Bill Clinton.

After Gray won, Bennett called about the attorney general job. “I said, ‘You’re doing yourself and the city the favor if you talk to Irv Nathan,’ ” Bennett said.

Gray agreed to meet. The “clincher,” he said, came during an interview with another candidate for the job, a former judge.

“He said . . . ‘Is it true that Irv Nathan is a candidate for this position?’ I said yes,” Gray recalled. “So the guy said, ‘If I were you, I’d hire him.’ ”

Nathan had never met Gray before the election, let alone supported his campaign. He declined to say whether he voted for him. “We have a system of secret ballots, and I’m going to keep that to myself,” said Nathan, who lives in the voting precinct in which Gray received his second-lowest percentage in last year’s primary.

He said he has since become a “big fan” of Gray, but he has not sought to be a public face of the administration, speaking only occasionally at mayoral news conferences. He has no media officer — “I’d have to give up a lawyer,” he said — and he has moved his main office from the John A. Wilson Building, where the executive and council staff reside, to the District office building at Judiciary Square, where more of Nathan’s lawyers work.

“I haven’t been pounding the table, but I’ve just been letting the facts speak for themselves,” he said. Schuelke said part of Nathan’s success is that he “doesn’t offend unnecessarily.”

That contrasts with his fiery predecessor, whom Nathan, for the most part, declined to discuss. “Peter Nickles is very intelligent, and he’s a very good lawyer, and I know he was doing what he thought was best for the District,” he said, adding that “a certain repair . . . needs to be made” with legislators, judges and staff.

Nathan belongs to a rarefied fraternity in the Washington legal community: a group of lawyers who cut their teeth as federal prosecutors in the post-Watergate years and went on to successful, lucrative careers in private practice. Many of them reunite each February for a trip to a Western ski resort — 19 years and running, Schuelke said.

Over his 40 years at Arnold & Porter, Nathan represented businesses and individuals. But his resume is notable for diverse government work.

As top deputy in the Justice Department’s criminal division from 1979 to 1981, Nathan oversaw the Abscam sting, which led to the conviction of a U.S. senator, five House members and several others for accepting bribes. The probe was controversial, with investigators criticized for using un­or­tho­dox methods and possibly entrapping politicians, but juries convicted each politician tried.

He also served as special counsel to congressional committees on two occasions, the second time for an investigation into the 2006 firings of nine U.S. attorneys by President George W. Bush’s Justice Department.

The firing scandal dominated his subsequent work for the House. With two top White House officials — Chief of Staff Joshua B. Bolten and counsel Harriet E. Miers — refusing to submit to congressional subpoenas, House Democrats debated whether to go to court to enforce them. If they sued and lost, the House’s ability to probe the executive branch could be permanently weakened.

Joseph N. Onek, who was a top legal adviser to then-House Speaker Nancy Pelosi (D-Calif.), said Nathan argued strongly to take the president to court. “If we lost, it would be a terrible precedent,” he said. “A group of us, with Irv in the lead, decided we should bring the case and had a decent chance of winning it.”

Nathan argued the case before U.S. District Judge John D. Bates, who ordered Bolten and Miers to comply with the subpoenas.

‘Most challenging job’

The attorney general position has given Nathan a new perspective on the city where he’s lived since 1974 with his wife, Judith A. Walter, an adjunct professor at Catholic University’s school of social work. In what he calls the “most challenging job I’ve had,” Nathan runs the city’s legal business, which continues to be troubled.

In May, U.S. District Chief Judge Royce C. Lamberth blasted city lawyers for a violation of discovery rules whose “exotic magnitude is literally unheard of in this Court.” The city continues to fight to extricate itself from court orders governing the delivery of some city services, some dating back decades. The office remains underfunded, with some lawyers handling double their usual caseloads.

Kristopher Baumann, head of the D.C. police officers union, which endorsed Gray and has clashed with city government, said Nathan’s tenure has been disappointing. “Their litigation behavior has not changed a bit,” he said. “It’s still obstruction.”

Still, Nathan has taken an aggressive stance toward government ethics issues, hiring a new ethics counselor who is developing a manual for city employees. He declined to investigate allegations of a job deal leveled against Gray by former mayoral candidate Sulaimon Brown, arguing that any findings would lack credibility. Last month, he told the D.C. Council that a proposed package of ethics reforms was too weak.

In his suit against Thomas, Nathan has put the city government on notice, even as some of Gray’s political allies murmur about whether Nathan was a politically smart choice.

“I’m prepared to defend everything that was done and every allegation that was made,” Nathan said. “We checked it many times.”