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 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 defense 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 then 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.

Focus on Ukraine

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 changes.

"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.

REUTERS