(CNN) Democrats have launched a critical new phase of their impeachment investigation by releasing first transcripts of witness depositions as they seek to build a public case that President Donald Trump abused his power in a rogue foreign policy operation in Ukraine.

Three committees investigating the scandal released transcripts and summaries of closed-door interviews with the former US ambassador to Ukraine Marie Yovanovitch and Michael McKinley, a former senior adviser to Secretary of State Mike Pompeo.

The move signals the start of a shift from closed-doors investigation of Trump's covert Ukraine policy into an attempt to convince Americans that he should be removed from office in dramatic televised hearings.

Signs that the release of testimony was imminent prompted Trump to issue a torrent of personal abuse against House Intelligence Committee chairman Adam Schiff, that was extraordinarily personal even by his own standards.

In the testimony released on Monday, Yovanovitch said she was shocked and felt threatened after learning that Trump attacked her on a call with Ukrainian President Volodymyr Zelensky as "bad news."

"I didn't know what it meant. I was very concerned. I still am," Yovanovitch said in the interview conducted on October 11.

The release of testimony came after it became clear that at least six executive branch insiders due to give depositions over the next few days would not show up. But this week could still have a sting in the tail.

Former national security adviser John Bolton , a fiery figure who was recently fired by Trump, is due to testify on Thursday -- though there are doubts over whether he will appear.

A Bolton sighting would spark a frenzy since other witnesses have testified that he decried Trump's decision to outsource Ukraine policy -- a caper one former senior official said he referred to as a "drug deal."

No one could be sure what Bolton might say. Would he stay loyal to his former boss or to the iconoclastic approach to foreign policy that steered him through a colorful career in Washington?

"I like John Bolton. I always got along with him," Trump said Sunday, misrepresenting his relationship with the renowned Washington bruiser in a possible sign of concern about his potential testimony. Asked whether Bolton speak up, Trump replied: " ... that's going to be up to him."

Democrats believe they have built a strong case that Trump pressured Ukraine to investigate a domestic rival -- Democratic 2020 candidate Joe Biden -- during weeks of depositions featuring career officials.

But if executive branch officials due to testify this week do not show up, they may reach a point when there is little to be gained by holding off a more visible phase of the inquiry. Party leaders believe they already have a strong enough case to go ahead without holdout witnesses, though more evidence could further bolster their confidence.

Long list of potential witnesses

All four White House officials who are scheduled to give depositions on Monday during the House's impeachment inquiry won't show up, as a source with knowledge of the situation tells CNN that National Security Council lawyers John Eisenberg and Michael Ellis will not testify.

The two officials will join Robert Blair , assistant to the President and senior adviser to the acting White House chief of staff Mick Mulvaney, and Brian McCormack, associate director for natural resources, energy & science at the Office of Management and Budget, in not testifying on Monday, CNN reported earlier. Energy Secretary Rick Perry, who was scheduled to appear Wednesday, will not participate in a closed door deposition, an Energy Department spokesperson said Friday.

Bolton's lawyer has said he will not appear without a subpoena. But it's not clear if he will testify even if he is served. And since Democrats have said they will not allow the impeachment drive to be bogged down in legal challenges like the one that could be necessary to force Bolton's compliance, they may move ahead without him.

Democrats have also warned that they will consider Trump administration attempts to block testimony by officials as a sign of obstruction that could be folded into articles of impeachment.

When the full House returns after a week-long break next Monday, two weeks will remain before the Thanksgiving recess -- a potential window for public hearings. In the normal course of events, the schedule for next week's committee business would be announced by the middle of this week.

Democrats hope to bring back important witnesses -- for example the top US diplomat in Kiev, Bill Taylor, who they believe will help them build a devastating picture of presidential malfeasance and unconstitutional behavior.

House Foreign Affairs committee chairman Eliot Engel laid out the theory of the case -- and a challenge to Republicans who want to shield Trump, in a television appearance on Sunday.

"No other president in American history has done something like that," Engel said on ABC News "This Week."

"He tried to essentially bribe a foreign power to interfere in US elections on his side to go after one of his political opponents," Engel argued.

New whistleblower gambit

In a new development over the weekend, lawyers for the intelligence community whistleblower who first raised alarm about Trump's dealings with Ukraine said their client was willing to answer written questions directly from Republicans without going through the Democratic majority.

The unidentified official's legal team has warned about threats to his safety. And Democrats now say their case is so strong that the whistleblower may not be needed to testify in public.

But Steve Scalise, the number two ranked House Republican, called for the whistleblower to testify to the committees investigating impeachment, complaining that "somebody behind closed doors, in secret" was "trying to take out a sitting president."

Rep. Jim Jordan, the top Republican on the House Oversight Committee, says written answers from the anonymous whistleblower are not sufficient, and said lawmakers need to hear from the whistleblower in person.

"Written answers will not provide a sufficient opportunity to probe all the relevant facts and cross examine the so-called Whistleblower. You don't get to ignite an impeachment effort and never account for your actions and role in orchestrating it. We have serious questions about this individual's political bias and partisan motivations and it seems Mark Zaid and Adam Schiff are attempting to hide these facts from public scrutiny. Last week's testimony raised even more concerns about the anonymous whistleblower and our need to hear from them, in person," Jordan said in a statement.

The Republican struggle to counter the facts of the case leaking out of the hearings was reflected in the scattershot defenses made by Trump loyalists on Sunday talk shows.

On CNN's "State of the Union," White House counselor Kellyanne Conway said she didn't know whether Trump withheld military aid to Ukraine -- the key matter at issue in the case.

But she stood by the White House response that there was no quid pro quo between Trump and the former Soviet state.

"President Trump never said to the Ukrainian President 'do this and you'll get your aid.' It is simply not here," she said, referring to a rough transcript of a July 25 call between Trump and the President of Ukraine.

"Quid pro quo, yes or no?" CNN's Dana Bash asked Conway.

"I just said to you. I don't know whether aid was being held up and for how long," Conway replied.

'Not impeachable'

Republican lawmakers tried out various lines of defense for the President, who has reportedly been frustrated that he is not being defended on the merits of the case.

"Look, if I believed everything the Democrats are saying I would still say this isn't an impeachable offense," said Oklahoma Republican Rep. Tom Cole on NBC's "Meet the Press." Cole is a nine-term veteran who once ran the House GOP's campaign arm and was a campaign consultant earlier in his career.

"Now we're going to do this on a phone call? I mean, I just don't think this rises to the level," he said.

The case that Democrats are building relates to far more than the phone call between the two Presidents. Leaks from depositions suggest that senior officials believe there was a long-running scheme to coerce Ukraine into investigating Biden and a conservative conspiracy theory that Kiev and not Russia interfered in the 2016 election.

But arguing that Trump's conduct does not meet the high crimes and misdemeanors standard for impeachment could be a viable path for Republicans caught between conservative grass roots support for the President and personal disquiet about his behavior.

Such an approach is not sitting well with Trump however, whose life code precludes any admission of wrongdoing and argued Sunday that the July call was "perfecto" and "totally appropriate."

"False stories are being reported that a few Republican Senators are saying that President Trump may have done a quid pro quo, but it doesn't matter, there is nothing wrong with that, it is not an impeachable event. Perhaps so, but read the transcript, there is no quid pro quo!" Trump tweeted on Sunday.