WASHINGTON (Reuters) - Three U.S. diplomats who expressed alarm about President Donald Trump’s dealings with Ukraine will serve as star witnesses when Democrats bring their impeachment case against Trump directly to the public with televised congressional hearings next week, lawmakers said on Wednesday.

In a preview of what is to come, lawmakers leading the probe released testimony that showed the top U.S. diplomat in Ukraine, William Taylor, believed a White House-led effort to pressure Kiev to investigate Ukrainian energy company Burisma was motivated by a desire to help Trump win re-election next year.

Hunter Biden, the son of former Vice President Joe Biden, served on Burisma’s board of directors. Joe Biden is a leading Democratic contender to face Trump in the 2020 presidential election.

“I understood the reason for investigating Burisma was to cast Vice President Biden in a bad light,” Taylor said, according to a transcript of last month’s closed-door testimony.

Taylor and George Kent, another career diplomat with experience in Ukraine, will testify before the House of Representatives Intelligence Committee on Nov. 13. Marie Yovanovitch, who was abruptly pulled from her post as U.S. ambassador to Ukraine in May, will testify on Nov. 15. All three have already testified behind closed doors.

Televised public hearings featuring U.S. officials testifying in Congress about alleged wrongdoing by Trump could crowd out other issues like the economy and immigration as voters turn their minds to the November 2020 election.

The hearings would be a likely prelude to articles of impeachment - formal charges - against Trump being brought to a vote in the House.

Democrats said they had enough material to move forward with public impeachment hearings even though some Trump administration officials have refused to cooperate.

“We are getting an increasing appreciation for just what took place during the course of the last year and the degree to which the president enlisted whole departments of government in the illicit aim of trying to get Ukraine to dig up dirt on a political opponent as well as further a conspiracy theory about the 2016 election that he believed would be beneficial to his re-election campaign,” Intelligence Committee Chairman Adam Schiff told reporters.

Schiff’s committee said later in the day that it had dropped an effort to force the cooperation of Charles Kupperman, a former White House deputy national security adviser who has refused to testify, saying it would take too long to resolve the issue in court.

Trump has blasted the House inquiry as a witch hunt and accused Democrats of unfairly targeting him in hope of reversing his surprise victory in the 2016 presidential election. In a tweet on Wednesday, Trump called the probe a “phony scam.”

Taylor has provided some of the most damaging testimony to date, telling lawmakers on Oct. 22 that Trump made the release of nearly $400 million in security aid to Ukraine contingent on Ukrainian President Volodymyr Zelenskiy publicly declaring Kiev would carry out politically motivated investigations demanded by Trump.

“That was my clear understanding, security assistance money would not come until the president (Zelenskiy) committed to pursue the investigation,” Taylor said.

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Taylor said he was told by Gordon Sondland, the U.S. envoy to the European Union, that Trump “was adamant” that Zelenskiy personally announce the investigations of Burisma and pursue a debunked conspiracy theory about the 2016 election.

‘THE RUSSIANS LOVING IT’

Taylor said he was worried that Zelenskiy would still be denied the security aid after announcing the investigations, damaging his credibility in Ukraine and the United States.

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“That was the nightmare,” Taylor said. “The Russians loving it. The Russians are paying attention.”

If the Democratic-controlled House votes to impeach Trump, the Republican-controlled Senate would then hold a trial on whether to remove Trump from office.

Senate Republicans have so far shown little appetite for ousting the president.

Trump’s White House is expected to bolster its communications team, bringing on former Florida Attorney General Pam Bondi and former Treasury Department spokesman Tony Sayegh to help make its case in the weeks to come, according to a senior administration official.

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 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.

(Graphic: The impeachment inquiry, here)