Correction: An earlier version of this column incorrectly spelled Gordon Sondland's name in one instance. This version has been corrected.

Ambassador to the European Union Gordon Sondland’s testimony makes exquisitely clear that the House, in exercising its impeachment power, is entitled to hear from a series of witnesses who have so far either refused requests to testify or not been summoned. They include, at a minimum: Vice President Pence, Secretary of State Mike Pompeo, acting White House chief of staff Mick Mulvaney, former national security adviser John Bolton, soon-to-be-former Energy Secretary Rick Perry and Rudolph W. Giuliani, the president’s personal lawyer.

While we’re at it, Sondland — and the committee — should be given access to documents that could help refresh his frustratingly spotty recollections. The administration’s high-handed order that witnesses not cooperate with the House probe and refusal to turn over relevant documents is an act of obstruction — and a likely count in the articles of impeachment. But the goal isn’t proving obstruction — it is getting at the truth.

The individuals listed above are firsthand fact witnesses in an inquiry whose seriousness was underscored by Sondland’s stark assessment: “Was there a ‘quid pro quo’? With regard to the requested White House call and White House meeting, the answer is yes,” Sondland testified. In addition, he said with regard to the $400 million in military aid for Ukraine whose release was being held up, “In the absence of any credible explanation for the hold, I came to the conclusion that the aid, like the White House visit, was jeopardized,” Sondland said.

Republicans have conjured up a bunch of smoke about the unfairness and unreliability of hearsay evidence, although hearsay rules do not apply in this setting, nor are they absolute. They have tried to discredit or play down the significance of some witnesses on the grounds that they had no direct contact with President Trump. Well, then let’s hear from those who did. The Watergate question is a cliche but one with particular pertinence here: What did they know, and when did they know it? Some fruitful potential areas of questioning:

Pence. Sondland testified that he spoke to the vice president before a Sept. 1 meeting with Ukrainian President Volodymyr Zelensky in Warsaw. “I mentioned to Vice President Pence before the meetings with the Ukrainians that I had concerns that the delay in aid had become tied to the issue of investigations,” Sondland testified. “I recall mentioning that before the Zelensky meeting. During the actual meeting, President Zelensky raised the issue of security assistance directly with Vice President Pence. The vice president said he would speak to President Trump about it.”

Pence’s office responded with a statement from his chief of staff, Marc Short, denying Sondland’s account. “The Vice President never had a conversation with Gordon Sondland about investigating the Bidens, Burisma, or the conditional release of financial aid to Ukraine based upon potential investigations. Ambassador Gordon Sondland was never alone with Vice President Pence on the September 1 trip to Poland. This alleged discussion recalled by Ambassador Sondland never happened.” Fine. If this is Pence’s account, it should not be a problem for him to answer questions about it under oath.

Pompeo. Preparing for that Sept. 1 meeting, which Trump was originally scheduled to attend himself, Sondland testified that he asked Pompeo “whether a face-to-face conversation between Trump with Zelensky could help” to get the aid released. “I would ask Zelensky to look him in the eye and tell him that once Ukraine’s new justice folks are in place … that Ze should be able to move forward publicly and with confidence on those issues of importance to Potus and to the U.S. Hopefully, that will break the logjam,” Sondland wrote to Pompeo. “Secretary Pompeo replied, ‘Yes.’” What did Pompeo know about why the aid was being held up?

Mulvaney. Sondland read from a July 19 email to Mulvaney, Pompeo, Perry and others in advance of Trump’s call with Zelensky: “He is prepared to receive Potus’ call. Will assure him that he intends to run a fully transparent investigation and will ‘turn over every stone’. He would greatly appreciate a call prior to Sunday so that he can put out some media about a ‘friendly and productive call’ (no details) prior to Ukraine election on Sunday.” Mulvaney responded: “I asked NSC to set it up for tomorrow.” What did Mulvaney & Co. know about the demands for an investigation?

Bolton. “Everyone was in the loop,“ Sondland testified. “On August 26, shortly before his visit to Kyiv, Ambassador Bolton’s office requested Mr. Giuliani’s contact information. I sent Ambassador Bolton the information directly.” Did Bolton speak with Giuliani? What was the conversation?

Perry. After Trump directed officials to “talk to Rudy,” Sondland testified, “Secretary Perry volunteered to make the initial calls with Mr. Giuliani, given his prior relationship.” The Energy Department issued a statement, saying Sondland "misrepresented both Secretary Perry’s interaction with Rudy Giuliani and direction the Secretary received from President Trump.” Great! Let’s hear from him.

Giuliani. There are too many questions for him to detail here, but you get the point: House members, and the American people, are entitled to hear this evidence.

The Constitution assigns the House of Representatives the sole power to impeach the president. This presumes the power to conduct impeachment inquiries, which presumes the power to call relevant witnesses — whether or not the president who is being impeached is happy about it.

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