Before and after the 2016 election — long before reading the report of special counsel Robert S. Mueller III — we knew that Mr. Trump had invited Russia to meddle. We knew that his son and campaign chairman had eagerly accepted a meeting with a lawyer associated with the Vladi­mir Putin regime promising compromising material about Mr. Trump’s opponent. We had learned, before the report, that this campaign chairman had taken millions in payments from Russia-aligned oligarchs in Ukraine while hiding those payments from federal authorities. And we knew that the Trump campaign had greatly benefited from the release of Hillary Clinton campaign emails stolen by Russian intelligence officers.

Yet when Mr. Mueller delivered his report — or, more accurately, when Attorney General William P. Barr delivered his misleading preview of the report — Mr. Trump claimed vindication. These previously known offenses against U.S. democracy, egregious as they were, tended to be overlooked.

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Now we are presented with another case of Mr. Trump soliciting foreign interference in the U.S. electoral process, perhaps more egregious in the sense that he was acting as president and not just as candidate. And again there is much we already know. We know, by Mr. Trump’s own admission, that he hectored the newly elected president of Ukraine to interfere in his own law enforcement apparatus in order to produce compromising material on former vice president Joe Biden or his son. We know, from earlier investigations, that there is no basis in fact for such an investigation. We know, thanks to Post reporting Monday, that just before this phone call, Mr. Trump suspended $400 million in U.S. military aid to Ukraine — aid that Congress had specifically appropriated; that the Pentagon recertified, after a White House inquiry, as necessary and consistent with U.S. foreign policy goals; and that Ukraine badly needs to defend against Russian aggression. We know that the aid remained suspended and that Mr. Trump continued to refuse Ukrainian requests for a presidential summit, until the apparent extortion attempt began to draw attention, by way of our Sept. 5 editorial and the subsequent launching of a House inquiry.

In recent days, thanks to reporting first by The Post and then by the Wall Street Journal, and the New York Times, we have learned that a U.S. intelligence official was so alarmed by Mr. Trump’s dealings with a leader or leaders in this matter that the official lodged a complaint. We know that the inspector general of the intelligence community found the complaint to be urgent and relevant enough to warrant forwarding to Congress, as required by law. And we know that the administration is blocking this transmittal, in apparent violation of law.

Congress therefore is right to demand, as an urgent matter, that the whistleblower complaint be shared with its intelligence committees, and that the whistleblower be allowed to testify. As part of the impeachment inquiry announced by House Speaker Nancy Pelosi (D-Calif.), the House is right to demand more information — not just the phone call transcript that Mr. Trump said he would release but also the president’s contemporaneous and current explanations for blocking the aid and denying a meeting, and testimony from Trump lawyer Rudolph W. Giuliani and from administration officials about their communications with Ukrainian officials.

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But Congress also should not forget what it already knows. Mr. Trump and his cringing Cabinet enablers are scrabbling furiously to obscure that truth by throwing out lies about the Bidens, pretending that the Trump phone call was normal and ethical and, maybe most contemptibly, impugning the patriotism of the whistleblower. Republicans in Congress might ask themselves how they would have responded if President Barack Obama had been caught importuning a foreign leader to concoct dirt on one of them and using the levers of American power to enforce the request. They and their Democratic colleagues should keep in the front of their minds what they already know.