The criticism of Elizabeth Warren illustrated her status as an emerging front-runner, while Joe Biden faced scrutiny over his son’s financial dealings overseas.
Mr. Kent told investigators he was cut out of Ukraine policymaking and told to “lay low” on it, after a May meeting orchestrated by Mick Mulvaney, the acting White House chief of staff.
Mr. Kent raised concerns to colleagues early this year about the pressure being directed at Ukraine by President Trump and his private lawyer, Rudolph W. Giuliani.
John R. Bolton, then the national security adviser, ordered an aide to report the machinations by the president’s personal attorney to a White House lawyer, House investigators were told.
The White House said President Trump condemned a fake video depicting him killing rivals and members of the news media. But Mr. Trump never mentioned it.
Fiona Hill, a former top adviser on the National Security Council, is prepared to tell House investigators that she objected to the ambassador’s removal.