As a series of diplomats have been interviewed in the probe, several of them detailing President Donald Trump's efforts to persuade Ukraine to investigate a political rival, many Republicans have been silent on the president's behaviour.
White House refusal to participate in the inquiry means no additional witnesses under administration purview will be permitted to appear in front of Congress or comply with document requests.
US President Donald Trump called the Ukranian leader to persuade him to launch an investigation into his Democratic rival, Joe Biden and his businessman son, according to a whistleblower complaint.
The Democratic-controlled House will examine whether President Trump abused his powers and sought Ukraine's help to smear former vice president Joe Biden, a 2020 Democratic front runner, House Speaker Nancy Pelosi said.
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