The impeachment inquiry is testing the Constitution's system of checks and balances as the House presses forward with the probe and the White House dismisses it as "illegitimate" without a formal vote of the House to open impeachment proceedings. A White House memorandum later showed that Trump repeatedly pressed Zelensky during the call to look into topics that could be politically useful for Trump in 2020. House Speaker Nancy Pelosi, D-Calif., has said she wants the committees to work "expeditiously" as Democrats gather evidence and prepare to make a final decision on whether to vote to impeach the president.
The impeachment probe was sparked by a whistleblower who revealed that Trump asked Ukraine's new president, Volodymyr Zelenskiy, on a July telephone call to investigate Biden's family and Ukraine's role of in the 2016 election that put Trump in office.
House Democrats have entered week four of their inquiry as they investigate President Donald Trump and other White House staffers, accusing Trump of pressuring Ukraine to investigate former Vice President Joe Biden and his son, Hunter.
New York Congressman Lee Zeldin, Republican member of the House Foreign Affairs Committee, questions which rules are governing the House Democrats" "fly-by-night' impeachment inquiry.
They have already heard from two: Kurt Volker, a former envoy to Ukraine, and Marie Yovanovitch, a former ambassador to Ukraine.
President Donald Trump's former top adviser on Russian Federation, who testified before three congressional committees Monday, objected "strenuously" to the firing of former ambassador to Ukraine Marie Yovanovitch, The New York Times reported.
"First of all, there doesn't need to be a quid pro quo", he responded, "but it is clear already I think from the text messages that this meeting that the Ukraine president sought was being conditioned on their willingness to interfere in the US election to help the President".
The Intelligence Committee also is scheduled to hear from Deputy Assistant Secretary of State George Kent and State Department Counsellor Ulrich Brechbuhl, a top aide to Secretary of State Mike Pompeo.
Lawmakers also may debate whether to seek to compel testimony from Rudy Giuliani, Trump's personal attorney who was pushing for Ukraine to investigate the Bidens. The deadlines for most of those requests are this coming week.
House Intelligence Committee Chairman Adam Schiff, D-Calif., who had originally suggested it was important for the whistleblower to provide closed-door testimony, backed off that stance during an appearance Sunday on CBS News's "Face the Nation".
"We are with this fly-by-night, making it up as we go along process where the American public is kept completely in the dark - except for what Adam Schiff wants to cherry-pick out to the American public".
Democrats have accused Trump of pressuring a vulnerable USA ally to dig up dirt on a domestic rival after withholding US$391 million (S$535 million) in United States security aid meant to help combat Russian-backed separatists in the eastern part of Ukraine.
Schiff said "it may not be necessary" to take steps that could risk disclosing the person's identity to find out more about the July telephone call.
And when the Washington Examiner has revealed that at least two Schiff staffers (one of them hired in the midst of the complaint-filing) worked with the whistleblower at the White House back when he was advising the Obama point man on Ukraine, Veep Joe Biden. "We have the best evidence of that".
His office earlier this month confirmed that the whistleblower approached House Intelligence Committee staff before the complaint was filed, effectively giving Democrats in the House advance warning of the accusations.