Bernie Sanders' doctors say the 78-year-old "has the mental and physical stamina" to be president. Cheryl Senter/AP hide caption

House Speaker Nancy Pelosi, D-Calif., holds hands with Rep. Debbie Dingell, D-Mich., as they walk to the chamber where the House begins debate on the impeachment charges against President Trump. J. Scott Applewhite/AP hide caption

House Judiciary Committee Chairman Jerry Nadler speaks to the media after the committee voted on the two articles of impeachment against President Trump on Dec. 13. Mark Wilson/Getty Images hide caption

House Judiciary Committee Chairman Jerry Nadler had adjourned Thursday without a vote on the articles of impeachment. Ranking member Doug Collins (in background) likened the move to a "kangaroo court." J. Scott Applewhite/AP hide caption

Department of Justice Inspector General Michael Horowitz arrives for a Senate Judiciary Committee hearing on the Inspector General's report on Capitol Hill on Wednesday. Andrew Harnik/AP hide caption

President Clinton in the Rose Garden of the White House on Dec. 11, 1998, before delivering a statement on the impeachment inquiry. J. Scott Applewhite/AP hide caption

House Speaker Nancy Pelosi, D-Calif., announced at the Capitol on Thursday that the House is drafting articles of impeachment against President Trump. Chip Somodevilla/Getty Images hide caption

Former White House national security aide Fiona Hill and David Holmes, a U.S. diplomat in Ukraine, return from a break to continue their testimony before the House Intelligence Committee on Thursday. Andrew Harnik/AP hide caption

"Was there a 'quid pro quo'?" Ambassador to the European Union Gordon Sondland said in his opening remarks to House Intelligence Committee."With regard to the requested White House call and White House meeting, the answer is yes." Win McNamee/Getty Images hide caption

Ambassador Kurt Volker, former special envoy to Ukraine, arrives to testify before the House Intelligence Committee on Capitol Hill on Tuesday. Susan Walsh/AP hide caption

Daniel Goldman, attorney and director of investigations with the House Intelligence Committee (second from left); committee Chairman Adam Schiff, D-Calif; ranking member Rep. Devin Nunes, R-Calif.; and Steve Castor, counsel for the minority, hold the first public hearing of the impeachment inquiry on Capitol Hill on Wednesday. Drew Angerer/Getty Images hide caption