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 am 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 chairperson 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 US 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 whistle blower 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 whistle blower 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 US 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.