Daniel Penny broke into a smile at midmorning Monday, hugging both of his lawyers in a Manhattan courtroom and getting a kiss on the cheek from one. Moments before, a jury forewoman had said Mr. Penny was not guilty of criminally negligent homicide in the death of a homeless subway passenger he had restrained in a chokehold last year.

As the celebration moved to a nearby bar, criticism of the district attorney, Alvin Bragg, exploded online. It came from Republicans like Vice President-elect JD Vance, Donald Trump Jr. and former Mayor Rudolph W. Giuliani, who praised the jurors and scorned Mr. Bragg for charging Mr. Penny, a former Marine.

On Friday, Mr. Vance said he had invited Mr. Penny to attend the Army-Navy football game on Saturday with him and called Mr. Bragg “New York’s mob district attorney.”

And so, as Mr. Penny’s star ascends on the right, Mr. Bragg, who faces re-election next year, finds himself in a familiar position: saying he had followed the law and his duty no matter the outcome as an internet storm raged around him.

Mr. Bragg, a Harlem native who is Manhattan’s first Black district attorney, has been a target for such fury since the first days of his term, when he promised a progressive approach to crime. It built to a fever when he charged President-elect Donald J. Trump with 34 felonies — and won a conviction on each charge.

After the Penny trial, Mr. Bragg said in a statement that “as with every case, we followed the facts and the evidence from beginning to end.” But he added that “prosecutors and their family members were besieged with hate and threats — on social media, by phone and over email.”

“Simply put,” he said, “this is unacceptable, and everyone, no matter your opinion on this case, should condemn it.”

Tumult comes with the job, said Cyrus Vance Jr., his predecessor, who is not related to the vice president-elect. But in recent years the 24-hour news cycle and the never-sleeping internet have made routine cases “more fraught more frequently,” he said.

“The office has always has been involved with tough cases and tough decisions,” Mr. Vance said. He added, “My guess is, the change in reporting has intensified the reactions to cases brought and not brought.”

It was cases possibly not brought that first made Mr. Bragg a focus of public ire. In his first week in office in 2022, Mr. Bragg told his staff to ask for jail time only for the most serious offenses — including murder, sexual assault and crimes involving major sums of money — unless the law required otherwise. The city was struggling to control a pandemic spike in crime, and the move created confusion and consternation in law enforcement circles.

But the most vociferous opposition came from conservative politicians after his office charged Mr. Trump. The former president portrayed him as part of a vast and sinister Democratic conspiracy as Mr. Bragg won his conviction for falsifying business records to cover up a hush money payment to a porn star. Mr. Trump has demanded the prosecution of people he blames for criminal and civil cases against him, including Mr. Bragg.

This summer, Mr. Bragg signaled that he would testify before Congress as Republican representatives sought to discredit the case. Since winning election this year for a second term, Mr. Trump has asked the court to dismiss his conviction. In a letter to the judge overseeing the case, Mr. Bragg’s office countered by showing a willingness to freeze sentencing while Mr. Trump holds office.

The district attorney’s office, with about 1,700 staff members, including approximately 600 prosecutors, has brought 36,000 cases this year, according to its data. In November, the office concluded 13 trials.

Mr. Bragg’s supporters have said that the politically charged cases have overshadowed good work, like mental health initiatives and the creation of a special victims division. Erin E. Murphy, a New York University law professor and Mr. Bragg’s close friend, said it is “frustrating.”

However, Mr. Bragg’s experience as a career prosecutor — working in the office of the U.S. attorney for the Southern District of New York and as a deputy New York attorney general — prepared him to take the condemnation in stride and ignore the political maelstrom, she said.

“The work he’s done has gone up against some of the most powerful political, economic, financial actors in our system,” she said. “He’s just well poised to just know what that feels like and what it entails.”

Mr. Bragg’s case against Mr. Penny stemmed from his encounter with another subway rider, Jordan Neely, on May 1, 2023. Mr. Penny, an architecture student, was on his way to the gym when he boarded an uptown F train. Mr. Neely, 30, who had struggled with his mental health for years, entered the car and began yelling about his hunger, wanting to return to jail and not caring about living or dying, according to witnesses, several of whom described his behavior as frightening.

As Mr. Neely strode through the car, Mr. Penny approached from behind and put him in a chokehold, taking him to the floor.

In the days after, as video of the two men struggling on the floor rocketed around the internet, protesters crowded onto the platform at the Broadway-Lafayette station, where the train had stopped, demanding charges against Mr. Penny.

Others quickly came to his defense, saying that he had acted to protect fellow passengers.

Vickie Paladino, a Republican city councilwoman from Queens, this week called for Mr. Trump’s incoming administration to launch a federal civil rights investigation of the prosecutor’s office. Mr. Bragg “has made this racial,” Ms. Paladino told “Fox & Friends First,” adding that the trials of Mr. Trump and Mr. Penny, who are both white, show the prosecutor’s office has a “vendetta.”

Maud Maron, a right-wing activist who has said she plans to run as a Republican for district attorney, said she would not have filed charges against Mr. Penny because he had acted in defense of others.

Mr. Neely would not have died had he been jailed for previous crimes, she said. Although incarceration is not “ideal or sometimes even a great way to deliver mental health services for drug treatment services, sometimes it’s the only way,” she said.

Mr. Penny’s case became a flashpoint in the debate over how New York handles crime and justice, homelessness and mental illness.

Some said the episode was representative of a string of high-profile crimes on the subways, many involving homeless and mentally ill people, and showed the city’s inability to protect residents. Others saw Mr. Neely as a symbol of a broken system that lets vulnerable people slip through the cracks.

That politicians seized on Mr. Penny’s case was unsurprising, said Maya Wiley, a civil rights attorney and former candidate for mayor. “In this era, political actors are deeply invested in what prosecutors are or are not doing,” she said.

Ms. Wiley, who said she met Mr. Bragg during their respective campaigns in 2021, called him a “straight shooter.”

Mr. Bragg had an “obligation to Neely and to the public” to look at the evidence and prosecute the case, particularly following a medical examiner’s findings that Mr. Neely died because of the chokehold, Ms. Wiley said. “Anything short of that would have been to fail to do the job appropriately,” she said.

But as Mr. Bragg’s office finishes one charged case, another is close on its heels.

At almost the same time as Mr. Penny was rejoicing on Monday, police officers in Pennsylvania arrested a suspect in the killing of a health insurance executive on a Manhattan street. The suspect, Luigi Mangione, has been charged with murder by New York prosecutors and they seek his extradition.

Already, the killing has garnered an impassioned response from Americans frustrated with the health insurance industry, with some making the defendant into a folk hero — and returning a polarized nation’s attention to the prosecutor’s office in Lower Manhattan.