Lt. Col. Alexander Vindman, a top Ukraine expert at the White House, on Tuesday is expected to blow holes through President Donald Trump's main defense about a July 25 phone call with Ukrainian President Volodymyr Zelensky.

Trump has said a whistleblower complaint accusing him of soliciting foreign interference in the 2020 election from Ukraine during the call is false and based on hearsay that makes the conversation seem more nefarious than it was.

But Vindman, who is scheduled to testify to Congress on Tuesday as part of its impeachment inquiry, will be the first official to talk to lawmakers who was actually on the call and therefore who has direct knowledge of it.

According to his opening statement, Vindman will testify that he felt Trump's actions could "undermine US national security" and that he twice reported his concerns about the president to John Eisenberg, the lawyer for the White House National Security Council.

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President Donald Trump has been unequivocal: The whistleblower who filed a complaint against him based on a phone call he had with Ukraine's president had no idea what they were talking about, as they weren't on it. The call was "perfect," and the whistleblower's complaint was based on hearsay that made the conversation sound more nefarious than it actually was.

But on Tuesday, a top Ukraine expert on the National Security Council is expected to blow holes through that defense.

Lt. Col. Alexander Vindman, a Purple Heart recipient who served in the Iraq War, is set to testify to Congress that he twice reported concerns about Trump's conduct toward Ukraine to an NSC lawyer.

More important — at least as it relates to Trump's defense — Vindman is the first official to testify who was on the July 25 phone call and therefore has direct knowledge of it.

According to the whistleblower's complaint, a White House summary of the call, testimony from current and former officials, and media reports, Trump repeatedly pressured Ukrainian President Volodymyr Zelensky during the call to investigate former President Joe Biden and his son Hunter and to look into unsubstantiated claims of Ukrainian interference in the 2016 US presidential election.

Days before the call, Trump ordered the US to withhold a nearly $400 million military-aid package to Ukraine. He didn't mention the aid during his conversation with Zelensky, but Trump reminded his Ukrainian counterpart that "we do a lot for Ukraine" and shortly after asked Zelensky to "do us a favor, though," and investigate the Bidens and the 2016 election.

Ukrainian officials learned of the aid freeze in early August, days after the phone call.

At the time, Trump loyalists like his personal lawyer Rudy Giuliani; the US's ambassador to the European Union, Gordon Sondland; and the US's special representative to Ukraine, Kurt Volker, were engaged in a shadow foreign policy campaign with Ukraine.

The effort primarily involved leveraging military aid, a White House meeting, and trade privileges for Ukraine in exchange for investigations meant to benefit Trump's reelection campaign.

Vindman's testimony is likely to make it even more difficult for Trump and his allies to defend his conduct, a feat they've already struggled with amid the cascade of testimony from US officials to House Democrats conducting an impeachment inquiry into Trump.

According to Vindman's prepared opening statement, he will tell lawmakers that he feared Trump's actions would "undermine US national security" and that he felt a "sense of duty" to speak out. He will also say the unofficial foreign policy that conditioned US aid to Ukraine in exchange for Ukraine acceding to Trump's personal demands was "harmful to US government policy."

Vindman's concerns began as early as July 10, more than two weeks before the phone call, when Sondland told Ukrainian officials during a White House meeting that a meeting between Trump and Zelensky was predicated on Ukraine launching "investigations" Trump wanted.

John Bolton, then the national security adviser, cut the meeting short and told a top Russia aide, Fiona Hill, to tell NSC lawyers about the conversation.

Vindman also reported concerns about Sondland's comments to the NSC's lead lawyer, John Eisenberg, according to his opening statement and testimony provided last week by Bill Taylor, the US's chief envoy to Ukraine.

But it was Trump's phone call with Zelensky on July 25 that prompted Vindman to fear the president would hurt national security.

"I did not think it was proper to demand that a foreign government investigate a US citizen," Vindman will testify, "and I was worried about the implications for the US government's support of Ukraine. I realized that if Ukraine pursued an investigation into the Bidens ... 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 reported his concerns to Eisenberg for a second time after the phone call.

He is one of nearly a dozen officials who have appeared before Congress so far, many of whom testified in direct defiance of orders from the White House or other federal agencies like the State Department.

Adam Schiff, the chairman of the House Intelligence Committee who is one of the three lawmakers leading the impeachment inquiry, said Monday that Democrats would not go to court to try to compel testimony from witnesses who refuse to testify.

Instead, Schiff said, House Democrats will use the White House's stonewalling and lack of compliance to add to potential articles of impeachment. Schiff's statement was a big sign that House Democrats likely have all the information they need to move forward on impeachment, and any additional testimony is icing on top.