WASHINGTON (Reuters) - An official on the White House’s National Security Council said he heard the U.S. ambassador to the European Union explicitly press Ukrainian officials to investigate Joe Biden and his son, according to a transcript released on Friday by Democrats leading the impeachment probe of President Donald Trump.

Alexander Vindman, a U.S. Army lieutenant colonel and Ukraine expert on the NSC, said Ambassador Gordon Sondland made it clear in a July 10 meeting at the White House that the investigations of the Bidens and Ukrainian gas company Burisma would have to be opened for Ukraine President Volodymyr Zelenskiy to get an Oval Office meeting with Trump.

“He was calling for something, calling for an investigation that didn’t exist into the Bidens and Burisma,” Vindman said. “My visceral reaction to what was being called for suggested that it was explicit. There was no ambiguity.”

His comments could give Democrats further ammunition in their push to determine whether Trump misused U.S. foreign policy to pressure Ukraine into carrying out a corruption probe into Democrat Joe Biden, a potential rival in the 2020 presidential election. The inquiry enters a critical phase next week when House committees hold impeachment hearings in public.

Vindman was among the U.S. officials in the White House Situation Room monitoring Trump’s call two weeks later on July 25 with Zelenskiy, a main focus of the impeachment probe.

He said he had no doubt that Trump demanded that Ukraine open the investigations in return for an Oval Office visit for the Ukrainian leader.

“This was about getting a White House meeting,” he said. “It was a demand for him to...fulfill this particular prerequisite.”

Vindman told lawmakers that he believes ties between the U.S. and Ukraine have been damaged by the administration’s actions.

“It undercuts U.S. resolve to support Ukraine and certainly puts a question into their mind whether they in fact have U.S. support,” he said.

Also on Friday, an attorney for Trump’s former National Security Adviser John Bolton sent a lawyer to House lawmakers that suggested Bolton knows about “many relevant meetings and conversations” concerning Trump’s actions toward Ukraine.

Democrats have sought Bolton’s testimony, but his lawyer, Charles Cooper, said in the letter that Bolton would not appear until a court rules on whether he should testify over objections from the White House.

With public hearings getting underway, House Republicans made a move to bolster their response to the impeachment probe.

House Minority Leader Kevin McCarthy announced that Representative Jim Jordan, an aggressive Trump defender, had been assigned to the House Intelligence Committee.

NO-SHOWS

Vindman’s account of the July 10 meeting at the White House was corroborated by Fiona Hill, the former senior director for European and Russian affairs on Trump’s NSC.

Hill testified she heard Sondland bring up Burisma at the meeting, according to a separate transcript of her testimony released on Friday.

Vindman’s testimony, in particular, highlighted fresh discrepancies of what occurred as recounted by Sondland, who told investigators he did not remember taking part in any effort to investigate the Bidens.

Slideshow ( 3 images )

Sondland, a Trump donor-turned-diplomat, acknowledged he encouraged Ukraine to investigate Burisma but said he did not know that Hunter Biden had served on the company’s board.

Vindman testified that Sondland told the July 10 gathering he coordinated the request with acting White House Chief of Staff Mick Mulvaney, who is Trump’s top aide as well as the director of the White House’s Office of Management and Budget.

Mulvaney was subpoenaed on Thursday night to testify but did not show up on Friday.

Mark Sandy, associate director for national security programs at OMB, also was called to testify and did not appear.

Slideshow ( 3 images )

The White House previously has said it would not cooperate with the congressional investigation, which was triggered by a whistleblower complaint about a phone call on July 25 between Trump and Zelenskiy.

Lawmakers wanted to question the two officials about their knowledge of OMB’s decision last summer to block nearly $400 million in security aid for Ukraine that had been approved by lawmakers.

Trump’s defenders say there is no evidence of him and the Ukrainian president engaging in a quid pro quo - or exchanging a favor for a favor - because the aid to Ukraine was released and Zelenskiy never explicitly promised to investigate Burisma, the Bidens, or any Ukraine involvement in the 2016 election.

A quid pro quo is not necessary, however, to prove high crimes or misdemeanors, which is the standard the U.S. Constitution requires for the impeachment of a president.

Earlier on Friday, Trump said he was unconcerned about the impeachment inquiry and criticized House Democrats for moving their inquiry into the public eye with open, televised hearings next week.

“They shouldn’t be having public hearings; this is a hoax,” Trump, who has denied any wrongdoing, told reporters.

The House committees conducting the inquiry are wrapping up the closed-door phase of their investigation before open hearings start next Wednesday with testimony from two diplomats who have been interviewed behind closed doors: William Taylor, the top U.S. diplomat in Ukraine, and Deputy Assistant Secretary of State George Kent.

Former U.S. ambassador to Ukraine Marie Yovanovitch, who was abruptly recalled by Trump in May, will testify on Nov. 15.