HOUSE DEMOCRATS took an important step in the impeachment process Thursday by approving a resolution that will open what, so far, have been closed hearings, with due-process guarantees for President Trump and his defenders. Predictably, Republicans who spent weeks demanding just such transparency and fairness voted unanimously against the resolution. Nevertheless, the open hearings that will now take place should allow Americans to hear and understand how Mr. Trump attempted to coerce the Ukrainian government into helping his 2020 reelection campaign. For their part, Republicans might have to take a stand on whether that behavior was proper.

Mr. Trump and his lawyers will have the opportunity to make statements, question witnesses and call their own, with the concurrence of the committee chairman. The rules are virtually identical to those adopted by Republicans when they pursued the impeachment of President Bill Clinton in 1998 — which makes their breathless claims about a “Soviet” process ludicrous as well as dishonest. One wrinkle is a provision that if Mr. Trump “unlawfully” refuses to make witnesses available or produce documents — as he has so far — the committee chair may restrict the president’s privileges. It seems only reasonable that Mr. Trump should not be allowed to call his own witnesses while blocking those sought by Democrats.

For now, it appears there are numerous honorable public servants willing to testify to Mr. Trump’s abuses, despite White House efforts to silence them. The latest on Thursday was Tim Morrison, a conservative Republican who served as senior adviser for Russia and Ukraine on the National Security Council.

According to The Post, Mr. Morrison offered firsthand confirmation of two crucial moments in Mr. Trump’s pressure campaign against the Ukrainians. He said he told the senior U.S. diplomat in Kyiv, William B. Taylor Jr., that the Ukrainians had been informed that U.S. military aid would be withheld until they announced an investigation of a gas company that employed Joe Biden’s son Hunter. In another conversation, Mr. Morrison told Mr. Taylor that Mr. Trump, during a phone call with Gordon Sondland, the U.S. ambassador to the European Union, had insisted that Ukrainian President Volodymyr Zelensky announce an investigation of the Bidens as well as of Ukraine’s alleged role in the 2016 election.

This is the sort of testimony that, when delivered in public, will make it difficult for Republicans to go on insisting there was no quid pro quo demand by Mr. Trump to the Ukrainians — and still maintain credibility with their own constituents. That’s why House Democrats are right to keep summoning witnesses such as Mr. Morrison, even if it extends the process beyond the Thanksgiving deadline they initially contemplated. While not allowing the White House to entangle them in lengthy court battles, Democrats should muster as much evidence as readily can be gathered.

When he is not slandering Democrats or the witnesses, Mr. Trump appears to be pushing Republicans to adopt the defense that there was nothing wrong with his actions — that he is entitled to use military aid, or the promise of an Oval Office meeting, to induce foreign governments to provide dirt on his possible 2020 opponent. So far, most Senate Republicans have shrunk from that position. Now that the process is going public, they will find it harder to hide.

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