"I now do recall."

With that stunning reversal, diplomat Gordon Sondland handed impeachment investigators another key piece of corroborating testimony Tuesday. He acknowledged what Democrats contend was a clear quid pro quo, pushed by U.S. President Donald Trump and his personal lawyer, Rudy Giuliani, with Ukraine.

Sondland, in an addendum to his sworn earlier testimony, said that military assistance to the Eastern European ally was being withheld until Ukraine's new president agreed to release a statement about fighting corruption as Trump wanted. Sondland knows that proposed arrangement to be a fact, he said, because he was the one who carried the message to a Ukrainian official on the sidelines of a conference in Warsaw with Vice-President Mike Pence.

"I said that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks," Sondland recalled.

The three-page update, tucked beneath hundreds of pages of sworn testimony by Sondland, Washington's ambassador to the EU, and former Ukraine special envoy Kurt Volker, was released Tuesday by House investigators as Democrats prepared to push the closed-door sessions to public hearings as soon as next week.

Trump has denied any quid pro quo, but Democrats say there is a singular narrative developing since his July 25 call with Ukrainian President Volodymyr Zelensky when he first asked for "a favour." That request, which sparked the impeachment inquiry, included a public investigation into Ukrainian activities by Democratic former vice-president Joe Biden and his son and Trump's allegations of Ukrainian interference in the 2016 U.S. election.

'More insidious'

Rep. Adam Schiff of California, the Democratic chairman of the intelligence committee, said the House panels conducting the inquiry are releasing the word-by-word transcripts of the past weeks' closed-door hearings so the American public can decide for themselves.

"This is about more than just one call," Schiff wrote Tuesday in an op-ed in USA Today. "We now know that the call was just one piece of a larger operation to redirect our foreign policy to benefit Donald Trump's personal and political interests, not the national interest."

Pushing back, Trump's Press Secretary Stephanie Grisham issued a statement saying the transcripts "show there is even less evidence for this illegitimate impeachment sham than previously thought."

In the transcripts and accompanying cache of text messages, U.S. diplomats are shown trying to navigate the demands of Trump and his personal lawyer, Rudy Giuliani, who they soon learn is running a back-channel U.S. foreign policy on Ukraine.

"It kept getting more insidious," Sondland told investigators, as the "timeline went on."

Sondland testified that he spoke with Secretary of State Mike Pompeo about Giuliani, "and Pompeo rolled his eyes and said: 'Yes, it's something we have to deal with."'

Memory jogged by others' testimony

In his revised testimony, Sondland, a wealthy businessman who donated $1 million to Trump's inauguration, says his memory was refreshed by the opening statements of two other inquiry witnesses, the top U.S. diplomat in Ukraine, William Taylor, and Tim Morrison, a European expert at the National Security Council.

The ambassador initially testified on Oct. 17 that he did not "recall taking part in any effort to encourage an investigation into the Bidens." He told investigators he didn't know that the Ukraine firm Burisma, that Trump wanted Ukraine to investigate, was linked to Joe Biden's son Hunter.

But in the weeks since a May visit to Kyiv for Zelensky's inauguration, Sondland and the other diplomats had been heavily involved in Ukraine policy and in text messages about what Trump wanted as they came to realize the military assistance was being withheld.

Kurt Volker, a former special envoy to Ukraine, is seen leaving a closed-door interview with House investigators on Capitol Hill on Oct. 3. (Jose Luis Magana/The Associated Press)

Volker and Sondland both testified they were disappointed after briefing Trump at the White House about the new leader of the young democracy who was vowing to fight corruption.

At a pivotal May 23 meeting, Trump "went on and on and on about how Ukraine is a disaster and they're bad people," Sondland testified.

Trump holds an alternative view, pushed by Giuliani, that it was Ukraine, not Russia, that interfered in the 2016 elections in the U.S., a theory counter to U.S. intelligence findings.

"'They tried to take me down.' He kept saying that over and over," Sondland recalled Trump saying.

Trump told the diplomats to work with Giuliani on Ukraine issues.

'I want nothing'

Over the time that followed, Volker and Sondland proposed to Zelensky's top aide, Andriy Yermak, that they draft a statement to be issued by Ukraine on potential interference with the U.S. political process. At Giuliani's urging, that statement needed to mention Burisma and the 2016 U.S. elections.

Pressed by investigators, Sondland testified that it would be improper for the U.S. to prompt Ukraine to investigate the Biden family. "It doesn't sound good," he said.

The statement was never issued, as Ukraine refused it. Volker said he told Yermak it was "not a good idea."

Questions swirled after a government whistleblower's August complaint about Trump's phone call with Zelensky.

By September, Sondland also told investigators, Trump was in a "bad mood" and nearly hung up on him when the ambassador asked what it was he wanted from Ukraine.

"I want nothing. I want no quid pro quo," Trump said, according to Sondland. "I want Zelensky to do the right thing."

More officials refuse to appear

As House investigators released more transcripts Tuesday, they also announced they want to hear from Trump's acting chief of staff and a top aide to Pence, reaching to the highest levels of the White House.

Pence spokeswoman Katie Waldman said the vice-president was unaware of the conversation that Sondland reported having with Yermak. She also said Pence was unaware of the ongoing back-and-forth over the statement, and that it never came up during his meeting with Zelensky.

Trump says the probe is illegitimate and the administration has resumed its efforts to block the inquiry as two more White House officials, an energy adviser and a budget official, declined to appear Tuesday before investigators, even after one received a subpoena.

With a year until the next U.S. president is elected, our political panel talks about how partisan politics and a possible impeachment will impact the campaign and the election outcome. 11:04

Meanwhile, investigators say they want to hear from Trump's acting chief of staff Mick Mulvaney because his news conference last month amounted to "nothing less than a televised confession" of Trump's efforts to have Ukraine investigate Democrats and Biden as the White House was blocking military funding for the Eastern European ally.

Trump says he did nothing wrong, and Mulvaney later walked back his remarks.

The White House has instructed its officials not to comply with the impeachment inquiry being led by House Democrats. Mulvaney is not expected to appear.

Republicans have been unable to deliver a unified argument against the impeachment probe, but one of them, Senate Majority Leader Mitch McConnell, said Tuesday he's "pretty sure" how it all will end.

"I don't think there's any question it would not lead to a removal," of Trump from the White House, McConnell said.

Most of those who have testified before the House panel are from the ranks of the State Department, including recalled U.S. Ambassador to Ukraine Marie Yovanovitch, whose testimony was released Monday. Diplomats have testified to the mounting concerns in the State Department over Trump's interest in having a foreign ally investigate Biden.

Sondland closed his addendum to the House investigators saying he may have had a second call with Trump, but has been unable to obtain phone records and "cannot specifically recall" if that was the case.