Cyrus R. Vance Jr., the Manhattan district attorney, seemed preoccupied when he sat down with two reporters last Monday. He already knew what the world would soon learn: his marquee prosecution — the sexual assault case against Dominique Strauss-Kahn — was falling apart. Privately, his aides had told him that they had discovered grave problems with the accuser's credibility.

The interview began, but before Mr. Vance was asked a question, he offered an unsolicited defense — not just of the Strauss-Kahn case, but of his overall stewardship.

"Ultimately," he said, "the success of a D.A.'s office— and of a D.A. — is measured not in individual cases, but over time."

"The cases you don't read about," he added, "define what the job of a D.A. really is."

But that job has grown increasingly tumultuous. Since Mr. Vance took over 18 months ago, morale in some parts of the district attorney's office has begun to sag, in part because of his firing of some prosecutors. Relations with one of the office's key partners, the Police Department, have grown tense at times, with the two agencies competing over many issues, including control of anticrime initiatives, according to officials on both sides.

Mr. Vance's predecessor, Robert M. Morgenthau, who became the pre-eminent prosecutor in the country while holding the post for 35 years, was once a close ally of Mr. Vance's, providing crucial support for his election in 2009.

Now, Mr. Morgenthau, 91, rarely if ever speaks to Mr. Vance.

Mr. Morgenthau has apparently become displeased with Mr. Vance's management style and his revamping of the staff that Mr. Morgenthau put together, according to people who know both men well. Mr. Vance's supporters attribute the criticism of his tenure to people who are unsettled by his efforts to reinvigorate and modernize an office that his supporters say had stagnated under Mr. Morgenthau. They pointed out that only after Mr. Vance became district attorney were prosecutors given smartphones.

Still, the second-guessing of Mr. Vance's leadership has intensified in the wake of a string of courtroom losses that culminated in the startling events last week, when prosecutors revealed their concerns about the honesty of the hotel housekeeper who accused Mr. Strauss-Kahn of sexual assault in May.

Even Gerald L. Shargel, a Manhattan defense lawyer who served on the finance committee for Mr. Vance's 2009 campaign, questioned how the case had been handled.

"What's most curious is hearing the line prosecutors saying early on that they had a strong case, a very strong case," Mr. Shargel said. "Obviously, they hadn't looked very hard. I have enormous respect for Cy as a prosecutor, but this is like a series of bad dreams."

A judge in Manhattan freed Mr. Strauss-Kahn from house arrest on Friday, and the case against him appeared to be collapsing.

In the weeks before that, Mr. Vance's office did not win rape convictions against two New York police officers accused of sexually assaulting a drunken woman (the officers were found guilty of lesser charges). And the most significant terrorism charges were dropped against two men accused of planning attacks against synagogues in the city, though serious counts remain.

Some of the most pointed complaints about Mr. Vance are emanating from the district attorney's office itself, according to numerous interviews with prosecutors and other officials. They spoke on the condition that their names not be used, saying they feared reprisals.

Several said they worried that cases were often pursued with an excessive focus on whether they would generate publicity. Some said Mr. Vance had taken away the discretion of midlevel prosecutors, sometimes to the detriment of cases.

Those two issues, some prosecutors said, contributed to the difficulties in the case against Mr. Strauss-Kahn, the former head of the International Monetary Fund who had been considered a leading candidate for the French presidency.

After Mr. Strauss-Kahn's arrest, the district attorney's office faced the question of whether to ask a judge to keep him in custody.

To do so, the office had to obtain an indictment within five days. The alternative was to agree to a bail package so that prosecutors could take their time investigating the case before deciding whether to indict, according to four people briefed on the matter.

In the end, Mr. Vance chose a quick indictment, drawing criticism that he moved before he knew of the accuser's background.

Prosecutors have said in court that they decided to seek the indictment and to keep Mr. Strauss-Kahn in custody to avoid the possibility of Mr. Strauss-Kahn's fleeing the country.

The case also unfolded as a rift had already developed between Mr. Vance and the chief of the office's sex crimes unit, Lisa Friel. She stepped down last week under circumstances that were not entirely clear. It did not appear that her decision was directly related to the Strauss-Kahn case.

Early on, Mr. Vance took the case away from the sex crimes unit and gave it to two other experienced assistant district attorneys.