The third day of the House impeachment inquiry's public hearing starts on Tuesday morning as four witnesses will testify over two consecutive hearings, kicking off a busy week for the investigation.

Jennifer Williams, an aide to Vice President Mike Pence, listened to that phone call and she is expected to give insight into the vice president's dealings with Zelenskiy.

In her closed-door testimony, Williams characterized the July 25 call as "unusual" - a comment that garnered criticism from the president, who referred to her as a "Never Trumper" Sunday on Twitter. In the call, Trump urged Zelensky to launch investigations into former Vice President Joe Biden and his son Hunter and into a debunked conspiracy theory about the 2016 USA presidential election.

The impeachment process has polarized US politics, with Democrats arguing Trump's behavior toward Ukraine amounts to an abuse of power, and Trump and some of his supporters calling it a "witch hunt".

Vindman tried to respond, but Wenstrup kept talking, until House Intelligence Committee Chairman Adam Schiff (D-CA) stopped him to allow Vindman time to finish his answer.

Investigators are scheduled to publicly question two witnesses on Thursday - David Holmes, a US diplomat stationed in Ukraine, and former National Security Council aide and Russian Federation expert Fiona Hill.

"You've never spoken to the president and told him advice on Ukraine", Turner said.

Kurt Volker, the former special envoy to Ukraine, who along with Sondland and Energy Secretary Rick Perry was part of the "three amigos" tasked by the president to handle Ukraine policy.

Ratcliffe said the shift in what Democrats are trying to impeach Trump for, "struck me as very odd because for the longest time this was all about quid pro quo, according to the whistleblower complaint, but after witness after witness began saying there was no quid pro quo or even that quid pro quo was not even possible, we saw a shift from the Democrats". Morrison will testify alongside Volker, who was also involved in White House meetings.

He did not name Mr Trump specifically, but the president has faced a backlash after tweeting attacks against a number of witnesses - some USA officials still working in his administration - who have agreed to give evidence.

Trump said Monday he would "strongly consider" testifying in writing to the House Intelligence Committee.

Impeachment investigators have released closed-door testimony from a USA official in Ukraine who overheard a call between diplomat Gordon Sondland and President Donald Trump this summer.

Mr Morrison's testimony contradicted much of what Mr Sondland told congressional investigators during his own closed-door deposition, which the ambassador later amended.

Tim Morrison, a White House aide with the National Security Council and who also heard the July phone call. He will likely be asked about a 26 July gathering at a Kiev restaurant where Sondland called Trump on his cellphone from a table on the outdoor terrace.

They include Gordon Sondland, the U.S. ambassador to the European Union, whose direct contacts with Trump are likely to be a main focus in the investigation of whether the president used United States aid as leverage to get Zelenskiy to dig up dirt on Biden.