REMEMBER HOW the Trump-Ukraine saga began: with President Trump corruptly pressuring the Ukrainian government to announce a bogus investigation into former vice president Joe Biden, tainting one of Mr. Trump’s key political rivals with the appearance of scandal. After the scheme came to light, Ukraine’s leaders never made Mr. Trump’s desired announcement. But, with the dishonorable help of some Republican members of Congress, the president may yet achieve his original goal: sliming Mr. Biden with absolutely no basis in fact.

“There is a mountain of evidence to suggest the Bidens’ behavior was harmful to the United States,” Sen. Lindsey O. Graham (R-S.C.) tweeted last Wednesday.

“Iowa caucuses are this next Monday evening,” Sen. Joni Ernst (R-Iowa) said earlier last week, following a presentation from Mr. Trump’s lawyers. “And I’m really interested to see how this discussion today informs and influences the Iowa caucus voters, those Democratic caucusgoers. Will they be supporting Vice President Biden at this point?”

As Congress has considered whether to impeach and remove Mr. Trump from office, Republicans rallying to his defense have argued that it is Mr. Biden who should be scrutinized. The former vice president’s son Hunter sat on the board of Ukrainian energy company Burisma at the same time his father pressed the Ukrainian government to fire its prosecutor general, who allegedly was investigating Burisma.

Seems fishy? Turns out there is nothing there. The prosecutor was not investigating the company — and it was official U.S. policy, in line with that of U.S. allies, international organizations and key members of Congress, that the Ukrainian prosecutor general should go. He was not pushed out because he was investigation corruption but because he was failing to do so. Congress was thoroughly briefed at the time about the prosecutor general and the then-vice president’s activities. And there is nothing to indicate Joe Biden did anything on behalf of his son in Ukraine.

Yet now, even though Mr. Biden fought corruption in Ukraine, Republicans insist he should receive scrutiny for allegedly enabling corruption there.

Mr. Graham’s behavior has been particularly shabby as he has threatened to misuse his chairmanship of the Senate Judiciary Committee to investigate the former vice president. He has announced a formal probe and asked the State Department for documents. “We’re not going to live in a world where only Republicans get looked at,” he said in December, indulging in a toxic (and unwarranted) display of grievance and implying that Mr. Biden’s demonstrably benign activities in Ukraine are equivalent to Mr. Trump’s corruption.

But there is no moral equivalence. There is no reasonable case against Mr. Biden. The case against Mr. Trump, on the other hand, is just as definitive in the opposite direction. Mr. Trump had no good reason to press Ukraine to investigate Mr. Biden, an American citizen whom U.S. authorities were not themselves investigating. He did so anyway, to no conceivable benefit to the United States. That Mr. Graham would suggest that the two belong in the same universe shows that he is so poisoned by partisanship, he either does not understand or has lost all moral direction.

Mr. Trump’s corrupt scheme to persuade the president of Ukraine to slander a political rival did not succeed. It is pathetic that U.S. senators would do the job instead.

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