(This Nov 12 story corrects date for April Trump-Zelenskiy call to April 21 from April 12 in final paragraph.)

WASHINGTON (Reuters) - Republicans and Democrats in Congress set battle lines on Tuesday ahead of televised hearings on the impeachment inquiry into President Donald Trump, drawing a vow from the Democrat leading the probe to thwart “sham investigations” into presidential hopeful Joe Biden’s family.

The hearings, due to start at 10 a.m. Wednesday and continue into next week, will push the inquiry into a critical new phase, with witnesses giving their first public testimony on whether Trump pressured Ukraine to target one of the president’s domestic political rivals with an investigation.

Representative Adam Schiff, the Democratic chairman of the House of Representatives Intelligence Committee, also said he would not allow the hearings to be derailed by a debunked conspiracy theory that Ukraine - not Russia - interfered in the 2016 U.S. election.

Some Republicans have defended Trump’s dealings with Ukraine by saying he was motivated by a desire to root out corruption in the Eastern European nation and get to the bottom of the election meddling four years ago.

Schiff’s comments in a note to committee members suggested he would not allow Republicans to call Biden’s son Hunter and the whistleblower who triggered the impeachment probe after a Trump phone call with Ukraine’s leader as witnesses. As leader of the Democratic majority on the intelligence panel, Schiff controls which witnesses testify.

The anonymous whistleblower set off the impeachment probe after Trump asked Ukrainian President Volodymyr Zelenskiy in a phone call on July 25 to investigate the Bidens.

The witnesses this week will be U.S. diplomats who voiced concern over Trump’s asking Ukraine to investigate Joe Biden, a top Democratic contender seeking to challenge Trump in the presidential election next year, and his son Hunter, who was on the board of a Ukrainian gas company, on unspecified allegations of corruption.

REPUBLICAN WITNESSES

Schiff also announced witnesses for hearings next week, some of whom were requested by Republicans, including Kurt Volker, a special envoy to Ukraine, Undersecretary of State for Political Affairs David Hale and White House National Security Council aide Tim Morrison.

Hunter Biden was not on the witness list, but a Republican aide said the party had not formally notified which of their witnesses had been rejected.

A statement from Schiff’s office said that he had “accepted all of the Minority requests that are within the scope of the impeachment inquiry.”

House Republicans met behind closed doors late on Tuesday to plot strategy over the hearings.

House Republican leader Kevin McCarthy said his party members would “just put the facts out there...there was no pressure. The Ukraine president said there was no pressure. There was nothing that Ukraine did (to) take any action and the money was released.”

Schiff, in his note to panel members, wrote that the inquiry and hearings “will not serve as venues for any Member to further the same sham investigations into the Bidens or into debunked conspiracies about 2016 U.S. election interference that President Trump pressed Ukraine to undertake for his personal political benefit.”

Schiff, who has become a key target of Trump’s attacks in recent weeks, added that the panel would not “facilitate” any effort to threaten, intimidate or retaliate against the whistleblower, a U.S. intelligence official.

Only intelligence committee members and staff would be allowed to ask questions in the hearings, said Schiff, adding that Republicans’ list of suggested witnesses was being evaluated and additional witnesses would be announced this week.

FILE PHOTO: U.S. President Donald Trump delivers remarks at a Veterans Day Parade and Wreath Laying ceremony in Manhattan, New York City, U.S., November 11, 2019. REUTERS/Andrew Kelly

‘OUT OF BOUNDS’

The investigation, formally launched six weeks ago by Democratic House Speaker Nancy Pelosi, has shadowed Trump’s presidency with the threat that he could be removed from office even as he seeks re-election next year.

“President Trump’s pressure campaign was ‘out of bounds,’ and every time he insists that it was ‘perfect’ he is saying that he is above the law,” Pelosi said on Twitter.

Trump is the fourth U.S. president to face impeachment. None were removed from office, although Richard Nixon resigned as he faced almost certain impeachment in 1974 over the Watergate scandal.

The White House has refused to cooperate with the probe and a number of administration officials have failed to show up to testify behind closed doors.

The New York Times reported on Tuesday that Trump has considered firing the intelligence community’s inspector general, Michael Atkinson, who first reported the whistle-blower’s complaints to Congress.

Trump, who appointed Atkinson in 2017, in recent weeks has continued to discuss with aides the possibility of firing him, The Times reported here, citing unnamed people familiar with the internal discussions.

For the past two weeks, House investigators have been releasing transcripts of interviews conducted behind closed doors with witnesses, including U.S. diplomats who have expressed concerns about dealings with Ukraine by administration officials and the president’s personal attorney, Rudy Giuliani.

Trump has intensified his attacks on the investigation ahead of the public hearings, which threaten to crowd out other issues like the economy and immigration as voters turn their minds to the November 2020 election.

Trump has denied any wrongdoing and branded the investigation a hoax. In a tweet on Tuesday, he said the Bidens should be called to testify. Biden has denied any wrongdoing related to Ukraine by him or his son.

House Democrats consider the open hearings as crucial to building public support for a vote on articles of impeachment - formal charges - against Trump.

If that occurs, the 100-seat Republican-controlled Senate would hold a trial. Republicans have so far shown little interest in removing Trump from office.

White House acting Chief of Staff Mick Mulvaney said in a court filing on Tuesday that he would not bring a lawsuit challenging a subpoena for his testimony, though he said that at Trump’s direction he would not cooperate with investigators.

The president also suggested on Tuesday that he would likely release the transcript of an April 21 conversation with Zelenskiy “before week’s end,” but gave no other details.