There has been a tendency over the years to romanticize the corruption of the old political machines. You often hear: “At least they got stuff done.” Politicians involved in what George Washington Plunkitt, the legendary 19th-century leader of New York’s Tammany Hall, called “honest graft” are cast as gleeful cynics who were refreshingly candid about their motivations.

But corruption is a dagger aimed at republican government. It turns what is supposed to be an institution devoted to the common good into an instrument of private gain. And the very worst forms of corruption turn what citizens count on to be neutral venues of legal adjudication and law enforcement into instruments of personal political power. Leaders of free governments cannot be above the law, and their political adversaries cannot be denied the law’s protections.

This is why the Justice Department’s move to intervene in the defense of President Trump in a defamation lawsuit brought by a woman who says he raped her should have friends of freedom shouting from the rooftops. For the U.S. government to substitute itself for Trump as the defendant puts the department at the disposal of one man, forces taxpayers to cover the costs of the president’s defense in what is a quintessentially private matter and is plainly aimed at preventing the public from learning more about the charges in the lawsuit before the election.

Yes, be very suspicious: Justice entered the scene only after a state judge in New York, as The Post reported, “had rejected Trump’s bid for a delay” in the case brought by the journalist E. Jean Carroll, thus opening the way for Carroll’s lawyers “to seek a DNA sample and an under-oath interview from the president.”

The flimsiness of Justice’s rationale for what the New York Times rightly labeled “a highly unusual legal move” would be shocking if anything that happened under Trump and Attorney General William P. Barr — who has clearly embraced the role of Trump’s consigliere — could shock us anymore.

In her suit, Carroll accused Trump of defaming her when he told the Hill newspaper in June 2019 that he could not have raped her because she was “not my type.” Trump also issued an official statement accusing Carroll of lying.

Justice claims it can stage what is in effect a takeover because Trump’s comments came while acting in his official capacity. Really? Some current and former Justice lawyers concerned about Barr’s maneuver are raising exactly the right question, the Times’s Alan Feuer writes: “Was it truly within the scope of the president’s duties to comment on the physical appearance of a woman who had accused him of rape?”

In a normal world, that question would answer itself. But we have left anything approaching the normal administration of justice. At a news conference on Wednesday, Barr acted as though this was no big deal, citing a 2006 ruling involving a Republican member of Congress as precedent, and he had the nerve to call the episode a “little tempest that’s going on . . . largely because of the bizarre political environment in which we live.” Never mind that Trump and Barr are principal architects of this environment.

Barr did not mention that if a federal judge rules in favor of Justice’s intervention, Carroll’s whole lawsuit would go away because Trump would come under the protection of the federal government’s “sovereign immunity.” Barr’s minions are, quite literally, trying to deny Carroll her day in court. At taxpayer expense.

We should worry about something else in our public life that has gone haywire because of Trump: He has faced so many credible accusations of assault and sexual misconduct that any new allegations get buried in some “been there, done that” file.

Trump and Barr are counting on this abuse of power to be tossed into this yellowing, bulging folder and dismissed. But a Justice Department that can arbitrarily protect a president can just as arbitrarily turn its investigatory and prosecutorial powers against the president’s political opponents.

“I think a lot of us are extremely alarmed, frankly, at the threat of autocracy,” Donald B. Ayer, a former deputy attorney general under George H.W. Bush, told Politico last month. Ayer, part of a group of Republican Justice Department appointees who endorsed former vice president Joe Biden, added of Trump: “He’s going to be unleashed if he gets a second term. I don’t know what’s going to stop him.”

Which is why voters need to resist becoming numb to forms of corruption that not only violate the norms of good government but also threaten to undermine equal justice and democratic rule. They are the only ones who can stop Trump.

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