Lt. Col. Alexander Vindman, a Ukraine expert assigned to the National Security Council, is testifying in the House impeachment inquiry Tuesday, offering new details on the push for investigations of President Trump’s political rivals and corroborating other witnesses with his firsthand account of the alleged attempt at a quid pro quo.

Vindman is the first impeachment witness to have listened in on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky, during which Trump said he wanted a “favor” after Zelensky broached defense cooperation between the U.S. and Ukraine, a key component of which is American military aid.

Vindman was listening from the White House Situation Room along with other NSC officials and members of Vice President Pence’s staff, he said in prepared remarks, and was so “concerned by the call” — and that the president’s request could be seen as “a partisan play” that could “undermine U.S. national security” — that he reported it to the NSC’s lead counsel.

Vindman’s prepared testimony touched a nerve with Trump. The president took to Twitter early Tuesday to deride the Iraq War veteran, who appeared for his testimony in uniform, calling him a “Never Trumper” and questioning his recollection of events.

“Supposedly, according to the Corrupt Media, the Ukraine call “concerned” today’s Never Trumper witness. Was he on the same call that I was? Can’t be possible!” Trump wrote on Twitter. “Please ask him to read the Transcript of the call. Witch Hunt!”

Trump’s tweet followed assertions on Fox News by a former Justice Department official, John Yoo, who claimed Monday night that Vindman might be engaging in “espionage” because he sometimes spoke with Ukrainian officials about strategy in his native tongue. Vindman and his family came to the United States from the Soviet Union when he was a small child.

Rep. Liz Cheney (R-Wyo.), the third-highest ranking Republican in the House, warned Tuesday that it was “shameful” for the GOP to question Vindman’s patriotism — or that of other witnesses testifying in the House’s impeachment probe.

“I think we need to show that we are better than that as a nation,” Cheney said. “It is shameful to question their patriotism, their love of this nation and we should not be involved in that process.”

Vindman’s testimony, which he gave under subpoena, directly challenged that of U.S. Ambassador to the European Union Gordon Sondland, a Trump appointee who met with impeachment investigators earlier this month. Sondland defended the president’s actions and told House investigators no one had raised concerns about them.

Sondland, in September text messages to the top American diplomat in Ukraine, Ambassador William B. Taylor Jr., said Trump had not engaged in a quid pro quo. Those text messages were provided to impeachment investigators by Kurt Volker, the Trump administration’s former special envoy to Ukraine.

In his meeting with lawmakers last week, Taylor laid out in meticulous detail how a shadow Ukraine policy involving Sondland and directed by Trump’s lawyer Rudolph W. Giuliani prioritized investigating Trump’s political rivals over U.S. national security interests. Taylor’s testimony has been held up by Democrats as potentially the most incriminating to date.

Vindman’s recollections, while narrower, illuminate key episodes in Taylor’s narrative with an even closer perspective: Vindman was either in the room or briefed personally after meetings by administration officials involved in the exchanges Democrats believe amounted to a quid pro quo.

Vindman also went to the NSC’s lead counsel with concerns about a July 10 meeting between Sondland, Volker, Energy Secretary Rick Perry, then-national security adviser John Bolton and senior Ukrainian officials. During the meeting, according to Vindman’s prepared statement, Sondland demanded that Ukrainian leaders deliver “specific investigations” to secure a meeting between Zelensky and Trump.

Vindman said he was told about that meeting directly by Sondland in the immediate aftermath of the event, according to his prepared remarks. During the previously scheduled debrief, “Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma,” a Ukrainian energy company, Vindman’s prepared testimony reads.

“I stated to Amb. Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push,” Vindman adds.

Sondland is already under pressure from some lawmakers to return to Capitol Hill due to discrepancies between his testimony and that of others like Taylor, who told investigators Sondland was aware Trump was leveraging a meeting and, later, the military aid for Ukraine on promises to conduct investigations.

Tim Morrison, the NSC official who told Taylor that Ukraine’s military aid was being held up to secure investigations into the role former vice president Joe Biden’s son Hunter had on Burisma’s board, as well as a debunked conspiracy theory involving a Democratic National Committee that was hacked in 2016, is expected to testify in the impeachment inquiry on Thursday.

Sondland appeared to demur during his closed-door deposition earlier this month about whether believed almost $400 million in military aid for Ukraine was being withheld to secure the investigations. But in recent days, Sondland’s lawyer Robert Luskin has told the Wall Street Journal his client believes — and told House investigators — that Trump’s refusal to meet with Zelensky until the Ukrainians promised to launch the investigations amounted to a quid pro quo.

Asked to respond to Vindman’s testimony, Luskin said his client would decline to comment.

Vindman’s prepared testimony does not address whether military aid was being withheld to secure U.S. elections; it just stresses that Sondland held back the promise of a phone call between the two heads of state until Ukraine pledged to conduct the investigations.

Sondland appeared on Capitol Hill again on Monday to review the transcript of his previous testimony in a secure facility, a courtesy afforded to all interviewees.

Vindman’s testimony also raises new questions about the role Bolton and his other senior deputies may play in the investigation as it proceeds. Bolton was infuriated by Sondland’s demands of the Ukrainians during the July 10 meeting, according to Vindman’s testimony and that of former NSC senior director for Russia and Europe Fiona Hill. Hill testified earlier this month that Bolton thought Giuliani was a “hand grenade” and wanted it known that he would not participate in a Ukraine policy he likened to a “drug deal” between Sondland and acting White House chief of staff Mick Mulvaney, who had convened the meeting.

The panels have thus far not subpoenaed Bolton for his testimony, though many Democrats believe he could be a powerful and incriminating witness against the president in a public hearing. But Bolton shares a lawyer with his former top deputy, Charles Kupperman, who petitioned the courts late last week to rule on whether he must comply with a congressional subpoena to testify in the impeachment probe.

Democrats, who are committed not to slow their probe by getting into protracted legal battles, have rejected Kupperman’s filing as legally baseless, with House Intelligence Committee Chairman Rep. Adam B. Schiff (D-Calif.) predicting Monday that the courts would “make short shrift” of the argument and force Kupperman to go before the panel.

That may happen sooner than initially appeared. A federal judge said Monday he wanted to hear from lawyers for the House, Kupperman and the Trump administration on Thursday afternoon.

Aaron C. Davis contributed to this report.