A Bronx woman who sucker-punched an anti-abortion activist during a street interview last spring is walking scot-free after the Manhattan district attorney, Alvin Bragg, failed to prosecute the violent crime.

Brianna J. Rivers, 30, was charged with second-degree assault, a felony, for the April attack on a self-described reporter for Live Action, a group that opposes legalized abortion. Despite a viral social media video of the incident, the charge was reduced to a misdemeanor and then dismissed in July when Mr. Bragg’s office failed to follow through with the case.

The Thomas More Society, a nonprofit public interest law firm, says it is preparing to file a civil lawsuit on behalf of the victim, Savannah Craven Antao.

“District Attorney Bragg’s shocking refusal to uphold justice only works to undermine confidence in the system, especially when our political climate has become as fraught as it is now,” senior counsel Christopher Ferrara said in a statement.

“Failing to prosecute these clear-cut charges sets a dangerous standard for how our society responds to violence against those engaging in democratic dialogue.”

Mr. Bragg’s office faced previous criticism over its decision to charge a veteran, Daniel Penny, with manslaughter in the death of Jordan Neely. Mr. Penny had applied a chokehold to subdue the irate homeless man who was threatening other passengers on a subway train.

Critics saw that case as a politically motivated prosecution of a good Samaritan, and a Manhattan jury eventually acquitted Mr. Penny.

The Thomas More Society’s lawsuit will be filed on behalf of Ms. Antao, who said in the statement provided by her lawyers that political violence should not be given a free pass.

“Millions of people saw my attack, an unprovoked assault, captured on camera while I was peacefully sharing my pro-life views and engaging in respectful dialogue. Even with indisputable evidence, District Attorney Alvin Bragg’s office quietly let the charges fade away, as if protecting people from violence was negotiable,” she said.

The incident was caught on video by Ms. Antao’s husband and quickly went viral.

“You don’t understand the magnitude of having a child,” Ms. Rivers can be heard saying in the woman-on-the-street-style interview before the punch is thrown.

Ms. Antao fires back, saying, “I’m not the one who admitted they would be okay with killing babies in foster care and killing children that have been abused.” She is referring remarks made by Ms. Rivers earlier in the interview.

“That’s not the point. What I’m saying is …,” Ms. Rivers begins to say before unexpectedly launching her fist into the interviewer’s face. She then strides off, yelling, “Suck my d—” to Ms. Antao’s husband.

The victim suffered two cuts to her head that required stitches.

Ms. Rivers was arrested a few days later and charged. She subsequently set up a fundraiser for her legal defense on GoFundMe and issued a half-apology on Facebook.

“To Savannah, I sincerely apologize but cannot sit around and allow you to continue pushing this one-sided narrative. I understand hands being put on someone is never the answer, but throwing rocks and hiding hands is worse,” she wrote at the time. 

“She knows the truth and she needs to release the footage in its entirety. Savannah is a professional antagonist not a ‘reporter’ and the truth will be told.”