Americans ought to thank Alan Dershowitz for his scintillating defense of President Donald Trump. Carried away on a crescendo of bluster, the retired Harvard law professor broadcast the true meaning of the acquittal preordained by crooked Senate Republicans: This president is exempt from any legal consequences, even if he seeks foreign assistance to rig his own election.

While the Dershowitz defense provoked an eruption of astonished guffaws and jeers — with “preposterous” the most frequent term of derision — it is significant precisely because it is so absurd and so blatantly disdainful of American constitutional values. It is also important to understand why the professor found himself advancing such a claim as senators prepare to vote on witnesses.

To align his closing argument with the White House and the Republicans, Dershowitz abandoned all pretense that the president didn’t shake down his Ukrainian counterpart to mount an investigation of Joe Biden and Hunter Biden in exchange for military aid and political support. Forced to acknowledge what everyone now knows about that scheme, Dershowitz instead insisted that even though Trump engaged in a quid pro quo, he is still immune from impeachment.

Prone as he is to extensive bloviation, Dershowitz dressed up his claim in dubious analogies and historical misinterpretations, with inferences drawn from the Mideast conflict and the Civil War. (If you enjoy listening to Dershowitz’s voice as much as he does, you can find the full riff on YouTube.) But with all the froth boiled away, the essence of his argument is unmistakable.

“Every public official that I know believes that his election is in the public interest. Mostly you are right. Your election is in the public interest,” said Dershowitz, asserting a judgment that is true of fewer than half the politicians upon whom he gazed. “If a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in impeachment.” He suggested that such a deal would only be impeachable if motivated by financial gain.

Could “something which he believes will help him get elected in the public interest” mean literally anything? Would shooting someone on Fifth Avenue be excused, too, or only “something” like extorting a hit job by a foreign power on a political opponent?

Eminent legal minds and Democratic senators pounced on the absurdity of Dershowitz’s statement — which blithely erased Trump’s corrupt abuse of power by attributing a pure motive to him. Dershowitz shot back, saying that the critics had willfully misinterpreted his words. But there is no way to parse them that is constitutionally palatable. They reek of a poison that would kill our system of free government.

Why would Dershowitz make this silly argument now? As fresh evidence of the criminal extortion scheme accumulates, including the possible testimony of former national security adviser John Bolton, this is the last defense left to the Trump lawyers. They know that the evidence overwhelmingly proves Trump is lying, and they don’t want that evidence paraded before the public on TV. Therefore, they must insist that the evidence wouldn’t matter. According to them, Trump is innocent even if he’s guilty, because he’s the president.

If that reminds you of someone you’d rather forget, it’s former President Richard Nixon, who blurted, “when the president does it, that means that it is not illegal.” Or, as Trump more crudely expressed in the dictatorial boast, “I have the right to do whatever I want as president.”

Both of these strutting autocrats were wrong. So are the Senate Republicans who seem prepared to violate their oath in advancing a criminal cover-up. Their craven complicity in these outrages will outlive them all, including Dershowitz.

Meanwhile, perhaps gazing up from a very hot place, Roy Cohn must be smiling. At long last, he has some very distinguished company.

To find out more about Joe Conason and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.