MORE THAN ever before in Donald Trump’s presidency, the U.S. political system faces a fundamental question: Does truth still matter? Our democracy depends on a positive answer. But the evidence of the past few days offers reason for worry.

In the matter of Ukraine, Congress and the public now have before them an indisputable set of facts. A rough transcript prepared and issued by the White House reveals repeated demands by Mr. Trump that Ukrainian President Volodymyr Zelensky launch politicized investigations, including of Joe Biden, who currently leads Mr. Trump by a wide margin in polling for next year’s presidential election. The transcript shows Mr. Trump first raising his request for “a favor” in response to Mr. Zelensky’s appeal to buy antitank weapons his army desperately needs. After Mr. Zelensky pledges to carry out the probes, Mr. Trump promises a prize he has been withholding for months: a White House visit.

Against this clear record of suborning support for his reelection campaign, Mr. Trump has thrown up a smokescreen of denials, insults — and blatant lies. Over and over, he and his personal lawyer, Rudolph W. Giuliani, have repeated the easily disproved claim that Mr. Biden sought to have a Ukrainian prosecutor fired to protect his son. Senior Ukrainian officials, including one of Mr. Giuliani’s own sources, have publicly stated that the story is false; multiple media investigations have definitively debunked it. Yet the president just goes on peddling it.

Other Trump lies are even more far-fetched: that Mr. Biden’s son Hunter “made millions of dollars from China”; or that President Barack Obama sought dirt on Mr. Trump from foreign leaders. Mr. Trump tweeted that Democrats “went to Ukraine and attempted to force the new President to do things that they wanted under the form of political threat.” Again, an easily established falsehood.

Through most of our political history, such a contest between documented facts and proven lies would be lopsided. When President Richard M. Nixon’s lies were exposed by White House tapes, his own Republican Party quickly turned against him. Yet in the past few days Mr. Trump’s propaganda has been embraced and echoed by GOP legislators and conservative media. Against the clear evidence of the transcript, they proclaim: “No quid pro quo!” Despite the abundant proof that neither Mr. Biden nor his son is guilty of wrongdoing in Ukraine, they reiterate dark insinuations about the former vice president’s “conflict of interest.”

What we could be left with, if Mr. Trump’s strategy succeeds, is a partisan standoff where there is no common ground of accepted facts. Like Russia’s Vladi­mir Putin, who uses lies to cloak his foreign aggressions, Mr. Trump wants to leave Americans with the impression that there is no discernible, objective truth about his behavior, only rival claims by bickering politicians and media in Washington that are best discounted or ignored. He doesn’t need to prove the Bidens are corrupt. He needs only to cast a shadow on the otherwise black-and-white evidence of the transcript.

There is really only one clear remedy for this, and that is for Republicans of integrity to stand up and acknowledge that Mr. Trump sought to pressure a foreign leader into digging up dirt on his foremost political opponent — and that it was wrong. A few have moved in that direction: Sen. Mitt Romney (R-Utah) and Rep. Michael R. Turner (R-Ohio) deserve some credit. But more must speak up. Those who remain silent, or worse, join in Mr. Trump’s lying game, will be complicit in the degradation of the U.S. polity.

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