President Trump’s entire rickety case rests on the dubious proposition that he was a corruption fighter, not a malicious president deadset on using his office to malign a political opponent. (Don’t we already know the president uses his office every day to attack critics — e.g., using antitrust powers to carry out his vendetta against CNN, and seeking to punish Amazon, whose founder and chief executive Jeff Bezos owns The Post?) For months now, his defense has been that he has an obsession with fighting corruption (except in his own government or governments led by dictators with whom he curries favor). He was not — do you understand, not?! — out to use his power to attack the political rival he fears most, former vice president Joe Biden.

But this is claptrap, as any reasonable person knows. In fact, his lawyers spent a good deal of Monday’s impeachment trial session falsely arguing that Biden’s pushing of Ukraine to fire a prosecutor (who was corrupt and whose ouster was sought by the United States and its allies) was just the same as Trump using his office to get a foreign government to investigate Biden, who had fired a corrupt prosecutor. Or something like that.

At any rate, Sen. Joni Ernst (R-Iowa), who has been running away from reporters and voters who dare ask whether she objects to using U.S. aid to extort a foreign government to interfere in our election, let the cat out of the bag. During a brief media availability on Monday, she rightly declared that the Iowa caucuses are set for next Monday. She then stepped into the political quicksand, confessing that she was “really interested to see how this discussion today informs and influences the Iowa caucus voters, those Democratic caucus goers. Will they be supporting VP Biden at this point?” Oops.

She is not supposed to be cheering in public for Biden’s demise as a result of already debunked conspiracy theories. Her confession however, like then-House Majority Leader Kevin McCarthy (R-Calif.) letting on that the Benghazi hearings would bring down Hillary Clinton’s favorability numbers, reminds us that the Trump extortion plan and the defense his legal team is conducting are very much — make that entirely — about undermining a rival who he became convinced would be his most dangerous opponent.

The Biden camp was all too happy to point out Ernst’s accidental confession:

In a written statement rebutting Trump’s lawyers, the Biden campaign also declared, “Here on Planet Earth, the conspiracy theory that [Trump lawyer Pam] Bondi repeated has been conclusively refuted.” The statement continued, “The New York Times calls it ‘debunked,’ The Wall Street Journal calls it ‘discredited,’ the AP calls it ‘incorrect,’ and The Washington Post Fact Checker calls it ‘a fountain of falsehoods.’ The diplomat that Trump himself appointed to lead his Ukraine policy has blasted it as ‘self-serving’ and ‘not credible.’ ” The statement concluded, "Joe Biden was instrumental to a bipartisan and international anti-corruption victory. It’s no surprise that such a thing is anathema to President Trump.”

Biden would be silly not to use all this to make his case in the final days leading to the Iowa caucuses. He is the one Trump fears. He is the one with the track record of corruption-fighting, which can be used to attack Trump’s own corruption. Defending and supporting Biden may be the ultimate refutation of Trump’s corrupt scheme. At least that is what Biden hopes the voters take away from this week.

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