The U.S. House Intelligence Committee is set to hear impeachment inquiry testimony Wednesday from a key actor in President Donald Trump's alleged effort to pressure Ukraine for incriminating information on his political rivals as Democrats continue to build their case for impeaching the U.S. leader.

U.S. Ambassador to the European Union Gordon Sondland will become the first witness with a direct line of communication to the  president to testify in public to the Democratic-led impeachment inquiry.

According to the testimony of other witnesses, Sondland spoke to Trump multiple times from mid-July to mid-September and could shed light on whether Trump abused his power by making U.S. security aid to Ukraine contingent on Kyiv's agreement to investigate Burisma, an energy company on which the son of former Vice President and political rival Joe Biden had served as a board member.

Sondland will likely face tough questioning from Democratic and Republican lawmakers after he revised his previous closed-door testimony to say there was a link between the withholding of $391 million in aid to Ukraine and the investigations Trump wanted. He testified in October that he knew of no preconditions to the assistance.

But on Nov. 4, the EU ambassador sent the congressional committee an addendum, saying statements from other witnesses had refreshed his memory about some conversations from early September.  In his revised testimony, Sondland said he now remembered that he had told an aide to the Ukrainian president that the United States "likely" would not send the much needed military aid until Ukraine agreed to investigate Burisma.

Sondland is set to testify in the morning, followed by career Pentagon official Laura Cooper and Undersecretary of State David Hale Wednesday afternoon. Former White House adviser Fiona Hill and career foreign service officer David Holmes are to testify Thursday.

It was a long and exhausting day in Washington Tuesday, where testimony in the impeachment inquiry lasted more than 11 hours.

Lawmakers on the Intelligence Committee heard from four witnesses – three of whom directly listened to Trump's July 25 telephone call with Ukrainian President Volodymyr Zelenskiy, when he asked Zelenskiy to investigate the Bidens.

Army Lt. Col. Alexander Vindman, the top Ukraine expert on the White House's National Security Council, said Trump's call was "inappropriate" and "improper."

"Frankly, I couldn't believe what I was hearing. It was probably an element of shock that maybe, in certain regards, my worst fear of how our Ukrainian policy could play out was playing out," he said.

Vindman said Trump's request to investigate Biden "had nothing to do with U.S. national security" interests and was not part of the talking points Vindman and others prepared for Trump ahead of the call.

Jennifer Williams, a foreign affairs adviser to Vice President Mike Pence, also testified Tuesday. She heard the call and said the Trump request for a Biden probe was "unusual" because it involved a "domestic political matter" and not foreign policy

Williams said in her 14 years as a foreign service officer, she has heard a lot of presidential phone calls, but nothing like what Trump was asking for.

After a break, the committee heard from two more witnesses.

Ambassador Kurt Volker, the former U.S. special envoy to Ukraine, had said in closed-door testimony that he did not see a link between Trump pressuring Ukraine for a Biden probe and Trump withholding nearly $400 million in military aid.

Volker said in Tuesday's testimony that after hearing other witnesses and seeing the transcript of the July 25 phone call, he now believes there was a connection. He said if he had seen things differently earlier, he would have raised objections.

Tim Morrison, who was the top director for European affairs on the National Security Council, testified Tuesday that he did not hear anything in Trump's July phone call that he would call illegal.

But Morrison said he does not think investigating Biden should be a "policy objective."

Trump and the Republicans allege that when Biden was vice president, he threatened to withhold loan guarantees to Ukraine unless prosecutors stopped a corruption probe into Burisma.

No evidence of wrongdoing by the Bidens has surfaced, and the allegations of Ukrainian election interference are based on a debunked conspiracy theory.

Democrats are focused on whether Trump froze military aid to Ukraine in exchange for Zelenskiy publicly committing to an investigation of the Bidens and some Democrats accuse Trump of bribery -- an impeachable crime.

Trump calls the hearings a huge scam and a witch hunt, insisting he did nothing wrong.

Read More