On Wednesday, the House will conduct the first public hearing of its investigation into President Trump's dealings with Ukraine.
The top diplomat in Ukraine, William Taylor, is one of two state department officials testifying in just a few hours.
Taylor is expected to tell lawmakers that he learned the White House set conditions for nearly half a billion dollars in aid that had been temporarily withheld.
Taylor will be joined by Deputy Assistant Secretary of State George Kent.
While Democrats said the witnesses will show the President tried to coerce a foreign nation, Republicans said the impeachment inquiry has been unfair from the start.
The White House said President Trump plans to watch at least part of the proceedings on television.