The New York attorney general was an hour into a Westchester County town hall, expounding on her view of her mission during President Trump’s second term — on democracy and the need to defend it, on courage and the need to display it — when a middle-age man stood up and told her she was going to prison for mortgage fraud.

The attorney general, Letitia James, did not visibly react. As members of her staff escorted the man from the room, she thanked him with a small smile, said the allegations were baseless and turned her attention to a less fired-up attendee who was taking the microphone.

The episode in Westchester last month neatly encapsulated the role Ms. James has staked out in recent years as one of Mr. Trump’s chief antagonists, and the risks of having done so. The audience member was referring to allegations that have become the subject of a criminal investigation by Mr. Trump’s Justice Department, whose leaders have rewarded the president’s allies and targeted his foes.

Ms. James has been one of the president’s nemeses since she brought a fraud lawsuit against him three years ago, leading to a half-billion-dollar penalty that Mr. Trump has appealed. And unlike many of his enemies, she has not fallen silent during his second term.

Her office has filed 21 lawsuits against him, working with other Democratic attorneys general to take aim at everything from Elon Musk’s slashing approach to the federal government to the administration’s sudden freezing of federal funds for states.

Many of the suits have successfully barred the White House from achieving its goals, at least in the short term. In May, for instance, a judge blocked Mr. Trump from moving forward with mass layoffs that would have gutted the U.S. Department of Education.