Mark F. Pomerantz, Carey R. Dunne and Michele Roberts, the former head of the N.B.A. players union, will launch a pro bono law firm, the Free and Fair Litigation Group.Last year, Mark F. Pomerantz and Carey R. Dunne were leading the Manhattan district attorney’s investigation into Donald J. Trump’s business practices.Now, they have turned their attention to a broader phenomenon that they say the former president represents: threats to democracy in the United States.Mr. Pomerantz and Mr. Dunne, who resigned last year when the district attorney decided not to seek an indictment of Mr. Trump, said they have formed a pro bono law firm that aims to stem the tide of anti-democratic policies proliferating around the country. The firm — the Free and Fair Litigation Group, which opens its doors this week — is also led by Michele A. Roberts, the former head of the union that represents professional basketball players.All three founders have extensive experience as litigators, and they plan to defend policies they see as just and bring lawsuits challenging those they believe are undemocratic, the three founding partners said in an interview. Their work will initially focus on voting rights, gun control and free speech.“As I see it, we’re now faced with not just one politician, but really with a national movement that’s aimed at rolling back decades of rights and constitutional principles,” Mr. Dunne said.In the two years since Mr. Trump’s false claims that the 2020 election had been stolen helped spark a violent riot at the Capitol, election denial has only grown within the Republican Party. Mr. Trump is once again a leading contender for president, and the House is in the hands of Republicans — many of whom voted against certifying President Biden’s election victory.Against that backdrop of deep political polarization, it remains to be seen how much of the new firm’s ambitious agenda can be accomplished, particularly if its cases reach a Supreme Court that has taken a sharp rightward turn.Michele Roberts, the former executive director of the N.B.A. players union, is an experienced litigator.Gabriella Demczuk for The New York TimesThe three founders will take no salary, and the firm will do all its work for free. They expect to hire a small staff of lawyers — no more than eight employees, including one who recently served as a federal prosecutor — and partner with a number of larger law firms. The firm, a nonprofit, will solicit outside donations from foundations and small donors alike.The new firm differs from larger groups like the American Civil Liberties Union and the Brennan Center for Justice, which conduct lobbying and research in addition to their work in court, because of its singular focus on litigation.The venture was a product of serendipitous timing: three busy lawyers who found themselves with nothing on their dockets in the spring of last year.Ms. Roberts, an experienced litigator, had just retired as executive director of the N.B.A. Players Association. Mr. Pomerantz, a well-known defense lawyer who also served as the criminal division chief at the U.S. attorney’s office in Manhattan, came out of retirement in early 2021 to lead the district attorney’s investigation into Mr. Trump. He resigned in February of last year, as did Mr. Dunne, another prominent litigator who oversaw the Trump investigation and successfully argued before the U.S. Supreme Court twice in the fight over a subpoena for Mr. Trump’s tax returns.Although Mr. Pomerantz and Mr. Dunne had begun to present evidence to a grand jury about the former president’s business practices by early last year, the new district attorney, Alvin L. Bragg, developed concerns about proving the case and decided not to seek to indict Mr. Trump at that time, prompting the resignations. The investigation, which began under the prior district attorney, Cyrus R. Vance Jr., is now continuing under Mr. Bragg, who also recently secured the conviction of Mr. Trump’s company.Carey Dunne won a Supreme Court decision that gave Manhattan prosecutors access to Donald Trump’s tax returns.Jefferson Siegel for The New York TimesWhile the new law firm currently has no plans to take on Mr. Trump directly, its mission was in some sense inspired by his influence over the Republican Party and the Supreme Court, to which he appointed three conservative justices.“Trump is obviously the poster boy for increased authoritarianism,” said Mr. Pomerantz, who has written a book about his time investigating Mr. Trump that Simon & Schuster announced on Wednesday would be published in February. “He personifies the problem, but he’s far from the only manifestation of the problem.”For his part, Mr. Trump has slammed Mr. Pomerantz publicly, calling him a “low-life attorney” who “is a ‘Never Trumper’ and a Hillary Clinton sycophant.”The firm’s first case involves gun control policies under attack in the wake of the Supreme Court’s ruling last year expanding the right to carry firearms outside the home. The firm is defending four Colorado towns, each with bans on carrying assault weapons in public, that were sued by a gun rights group after the court’s decision. The case is scheduled to go to trial this fall.Measures like Florida’s “Stop W.O.K.E.” law, which limits talk of race, gender and nationality in schools and the workplace, are also of interest at the new firm. It has begun examining the possibility of bringing a First Amendment lawsuit focused on similar laws in other states that prohibit diversity training in the workplace.The firm is also developing plans to challenge Florida’s arrest of a number of people with criminal histories who were able to register to vote in the 2020 election even though their past convictions should have barred them from doing so. Although criminal charges against some of those people have been dismissed, the firm is researching the possibility of suing the state for having violated the Voting Rights Act, arguing that the arrests discourage legal voting by people with criminal convictions.“It’s just disgraceful,” Ms. Roberts said, adding that the case had hit home for her as someone who is concerned with voting rights and with “legislative changes to election laws in various states.”To litigate the cases, the firm will turn for support to a roster of prominent law firms and advocacy groups.In the Colorado gun control case, the firm is working with Everytown for Gun Safety, the group founded by the former New York City mayor Michael R. Bloomberg, as well as Davis Polk, Mr. Dunne’s former firm. On the Florida voting rights case, they have met with a group called Protect Democracy, a nonprofit founded by lawyers from the Obama White House, as well as Paul Weiss, where Mr. Pomerantz was a partner for many years.Mark Pomerantz and Mr. Dunne led the Manhattan district attorney’s inquiry into Mr. Trump until they resigned last year.David Karp/Associated PressThe outside firms are providing their resources on a pro bono basis. The new firm’s board of directors will also include a number of boldfaced names from the criminal justice world, including Tali Farhadian Weinstein, a former general counsel at the Brooklyn district attorney’s office who was a Democratic candidate for Manhattan district attorney in 2021.Free and Fair’s executive director will be Danny Frost, a lawyer who served as a senior adviser and spokesman in the district attorney’s office under Mr. Vance, when Mr. Dunne and Mr. Pomerantz were working on the Trump investigation.Thus far, the firm has mostly accepted donations from friends and professional acquaintances but in the coming months will ramp up their fund-raising now that the I.R.S. has certified the firm as a nonprofit.Originally, the principals had expected that getting authorized for tax-exempt status would take half a year. Then they learned that they could make an emergency application to the I.R.S., “but only if what you’re providing is so desperately needed by the country that you can claim emergency treatment status,” Mr. Dunne said.They filed their application in October. Within 14 days, they had received the emergency approval. More