President Trump’s lawsuit against New York’s attorney general, Letitia James, deserves, in our view, a serious hearing. Mr. Trump complains that she’s investigating him for political purposes. That truth is so plain that even the New York Times has been warning against it. This is a moment for the robed sages to declare that they will not permit their hallowed courtrooms to be used for politics.

It’s hard to think of a more important principle. We carry no brief for Mr. Trump or any of his businesses. And we fully appreciate what a formidable opponent Ms. James is as a political operator. Without ever filing a charge, she took down, in Andrew Cuomo, the man standing athwart her own ambitions for the governorship. It’s no small thing, though, that Mr. Trump was in her sights when she was but a candidate.

During the race for attorney general in 2018, Ms. James told Democratic primary voters she was “closing in on him.” After winning the nomination, she credited her victory to her aggressive criticism of the President. Her win, she said, “was about the people but most importantly it was about that man in the White House who can’t go a day without threatening our fundamental rights.” She vowed to “stand up and fight back.”

Ms. James referred to the president as “illegitimate.” She described herself to the Atlantic as “a street fighter, and that’s the only thing that Donald Trump respects and understands.” Mr. Trump “should be charged with obstructing justice,” CNN quoted her as saying in a campaign video, and she intended to “follow the money” because she believed President Trump was engaged in the “pattern and practice of money laundering.”

The vehemence of Ms. James’s anti-Trump statements startled the Times, at least as we read its story. Her “strident attacks on the president,” it speculated, could undermine her ability as attorney general to prosecute cases “against Mr. Trump, his family members or their business interests.” The Times cited a former prosecutor as reckoning that Ms. James’ rhetoric might create the impression among judges of “an individualized political vendetta.”

The ex-prosecutor quoted by the Times, Daniel Goldman, called it “essential that prosecutors maintain their neutrality and an objective view of the facts and the evidence, no matter the politics involved.” When the Times sought Ms. James’ reaction to these concerns, she deflected the criticism as “assumptions and misconceptions” based on “her race and gender” and, the Times reported, warned that she had long been “underestimated.”

Ms. James’ comments also raised the hackles of a noted lawyer and civil rights advocate, Harvey Silverglate. He said at the time that “to begin an investigation with the target already selected is, in my view, a form of corruption of the legal system.” Mr. Trump previously said Ms. James “openly campaigned on a GET TRUMP agenda.” That observation animates Mr. Trump’s lawsuit demanding an end to her civil investigation.

This is a moment to pull out the well-thumbed copy of the most famous speech ever given about prosecutors. It was delivered in 1940, by Justice Robert Jackson. He spoke to United States Attorneys of the danger that a prosecutor “will pick people that he thinks he should get, rather than pick cases that need to be prosecuted.” In that case, he said, “the real crime becomes that of being unpopular with the predominant or governing group.”

Ms. James’ patently-political, launched-from-the-hustings pursuit of the president is a classic example of what Justice Jackson had in mind. She launched her campaign for attorney general by boasting of her intent to get Mr. Trump. She was not yet attorney general and so hadn’t officially gathered a particle — nay, an electron — of evidence. Yet she launched her pursuit of him from the stump. How in the world can she deny his complaint that it is political?