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On Tuesday, the National Security Council’s leading Ukraine expert, Army Lieutenant Colonel Alexander Vindman, will testify before the House committees leading the impeachment inquiry, becoming the first current White House official to provide testimony on President Trump’s July 25 call to Volodymyr Zelensky, in which he pressured the Ukrainian president to investigate Hunter Biden in exchange for the release of military aid.

In his opening statement, published on Monday night, Vindman reports that he twice expressed his objections to superiors, concerned that the administration’s actions would “undermine U.S. national security.” His first complaint came after a July 10 meeting in which U.N. ambassador Gordon Sondland “emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma,” referring to a debunked conspiracy involving Ukrainian interference in the most recent U.S. presidential election and Hunter Biden’s dealings with the energy company Burisma, for which he was cleared of any wrongdoing. The second complaint came after Trump’s call to President Zelensky on July 25:

“I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government’s support of Ukraine. I realized that if Ukraine pursued an investigation into the Bidens and Burisma it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained.”

Vindman, a Ukrainian-American immigrant who was wounded by a roadside bomb while serving in Iraq, was also concerned by “outside influencers promoting a false narrative of Ukraine inconsistent with the consensus views” inside the intelligence agency. According to all indications, — and additional documents reviewed by the New York Times — he’s referring to Rudy Giuliani’s crack team. Though Vindman intends to testify that he is “not the whistle-blower,” his testimony suggests he may be one of the aides that the whistle-blower referenced discussing the matter with in their initial complaint.

Aside from corroborating the testimony of ex–NSC official Fiona Hill — which established the national-security apparatus’s leeriness regarding Trump’s actions and Giuliani’s presence — Vindman’s account, that of the first current White House official to appear before the House impeachment inquiry, also shows that the president’s efforts to suppress testimony aren’t going to succeed beyond his true believers or those on his payroll. Vindman won’t be the last White House official to testify this week. On Thursday, as the House intends to vote on impeachment proceedings, NSC official Tim Morrison will also appear before the inquiry. Morrison, who was reportedly the first official on the July 25 call, intends to testify even if the White House makes an effort to block him.