DURING THE House Judiciary Committee’s first impeachment hearing Wednesday, legal scholars said President Trump was guilty of grave abuse of his office and warned that declining to impeach him would invite more brazen behavior from Mr. Trump and future presidents. A dissent came from George Washington University law professor Jonathan Turley, whom Republicans called to testify. He avoided defending Mr. Trump’s actions on their merits. But he argued they were not impeachable crimes and insisted the Democrats should spend more time gathering evidence before moving the process forward. He was half-right.

Mr. Turley incorrectly played down the gravity of the allegations against Mr. Trump and the strength of the existing record. The public already knows that, as Ukrainian President Volodymyr Zelensky sought military aid and a White House meeting, Mr. Trump asked for “a favor”: investigations based on false premises that would have helped Mr. Trump politically. Only after Mr. Zelensky assured Mr. Trump that investigations would proceed did Mr. Trump indicate a White House meeting would be possible. All of this is in black and white in the rough transcript of a July 25 call the White House released. Then there is the recent testimony of Trump administration officials indicating that Mr. Trump held up military aid, not just a White House meeting, to press for his desired investigations.

Three prominent law professors testified Wednesday that conditioning official acts, in this case a White House meeting and, very likely, the release of military aid, on the receipt of something of value, in this case flimsy investigations the president wanted, is an example of the sort of bribery the Constitution specifies as grounds for impeaching the president. Mr. Turley’s protests that such behavior might not meet the technical definition of criminal bribery missed the point: Even if Mr. Turley were right, the House is prosecuting the president under a constitutional standard, not a federal statute.

Yet this damning tale still lacks the accounts of several critical witnesses. Acting White House chief of staff Mick Mulvaney could demystify why Mr. Trump halted the military aid. Former national security adviser John Bolton and Secretary of State Mike Pompeo could describe what they heard from the president. The problem is that Mr. Trump’s lawless embargo of the House’s impeachment proceedings has prevented these key witnesses from testifying and the courts might take time before deciding on the House’s appeals.

We do not blame the Democrats for feeling frustrated. And they may rightly think they already have all the evidence they need. But if they could strengthen the case, they should do so. Because the stakes are so high, extra time may be justified if it results in testimony from administration witnesses. This also might convince more Americans that the impeachment process has been conducted thoroughly.

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