Public hearings in the impeachment inquiry into Donald Trump will begin next week, House Intelligence Committee Chairman Adam Schiff said on Wednesday, with three American diplomats who expressed alarm about the president's dealings with Ukraine set to appear.

Who will testify?

The top U.S. diplomat in Ukraine, William Taylor, and George Kent, another career diplomat with experience in Ukraine, are set to feature in the first hearing on November 13. Marie Yovanovitch, who was recalled as ambassador to Ukraine by Trump in May, is expected to give her testimony in the second hearing on November 15.

All three have already testified behind closed doors.

In a preview of what is to come, lawmakers leading the probe released testimony that showed 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.

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 of Representatives.

How much evidence has emerged?

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," Schiff said.

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 October 22 that Trump made the release of nearly 400 million U.S. dollars 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.

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

Giuliani hires his own attorneys

Rudy Giuliani, a personal lawyer for Trump and a central figure in the congressional impeachment inquiry, has hired lawyers to represent him as he faces scrutiny by federal prosecutors in New York.

The former New York mayor and federal prosecutor tweeted that he is being represented by Robert Costello, a former New York federal prosecutor, and Eric Creizman and Melissa Madrigal of the law firm Pierce Bainbridge.

Prosecutors in the Manhattan U.S. attorney's office are examining Giuliani's interactions with two men arrested in October for allegedly illegally funneling money to a pro-Trump election committee and other political candidates, according to another person familiar with the matter.

Giuliani has not been accused of criminal wrongdoing.

Giuliani is a central figure in a fast-moving impeachment inquiry into whether Trump abused his office for political gain.

As part of their impeachment inquiry, House Democrats have subpoenaed Giuliani's communications in connection with his efforts to push the Ukraine investigation.

Trump has derided the congressional probe as a political smear, and Giuliani has said he would not cooperate.

(With input from AFP, Reuters)