WASHINGTON   -    The impeachment inquiry into U.S. President Donald Trump will reach a critical juncture on Wednesday when lawmakers launch their first televised public hearings, marking a new, high-stakes phase of proceedings that could determine the fate of his tumultuous presidency.

Democrats leading the U.S. House of Representatives probe have summoned three U.S. diplomats – all of whom have previously expressed alarm in closed-door testimony about Trump’s dealings with Ukraine - to detail their concerns this week under the glare of wall-to-wall news coverage.

Trump’s fellow Republicans, who will also be able to question the witnesses, have crafted a defence strategy that will argue he did nothing wrong when he asked Ukraine’s new president to investigate Joe Biden, a former vice president and key 2020 re-election rival.

Both sides will be playing to a sharply polarized electorate as they move deeper into a six-week-old investigation that has shadowed Trump’s presidency with the threat of being removed from office even as he campaigns for a second term. It has been two decades since Americans last witnessed impeachment proceedings against a president, and these will be the first of the social media era. Republicans, who at the time controlled the House, brought impeachment charges against Democratic President Bill Clinton in a scandal involving his sexual relationship with a White House intern. The Senate ultimately voted to keep Clinton in office.

Though no president has ever been removed from office by impeachment, that has not deterred Democrats, who are looking into whether Trump abused his power by withholding nearly $400 million in security assistance to Ukraine to pressure the vulnerable U.S. ally. The focus is a July 25 phone call, in which Trump asked Ukrainian President Volodymyr Zelenskiy to open a corruption investigation into Biden and his son and into a discredited theory that Ukraine may have meddled in the 2016 U.S. elections.

Trump has denied any wrongdoing, derided some of the current and former U.S. officials who have appeared before committees as “Never Trumpers,” and branded the investigation a witch hunt aimed at hurting his re-election chances.

Hours before the hearing, he tweeted comments from conservative media supporters calling the proceedings a sham. “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,” House Speaker Nancy Pelosi said on Twitter, referring to how the Republican president has described his actions in the Ukraine saga.

Trump also suggested on Tuesday that he would likely release the transcript of an April 12 conversation with Zelenskiy this week but gave no other detail. It was the July phone call that prompted an anonymous whistleblower to set off the impeachment probe.

With a potential television audience of tens of millions, two witnesses – William Taylor, top U.S. diplomat in Ukraine, and Deputy Assistant Secretary of State George Kent – will be sworn in before the House Intelligence Committee on Wednesday. Lawmakers leading the probe released transcripts of closed-door testimony last week showing that 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.

Taylor testified he had been concerned to learn that security aid to Ukraine, as well as a White House meeting between Trump and Zelenskiy, had been delayed for political reasons. Kent said he had been alarmed by efforts by Trump’s personal lawyer, Rudy Giuliani, and others to pressure Ukraine. He said Giuliani - who Democrats have accused of conducting a shadow foreign policy effort in Ukraine – had conducted a “campaign full of lies” against Marie Yovanovitch, who was abruptly pulled from her post as U.S. ambassador to Ukraine in May.