House lawmakers, who have interrogated a string of diplomats at the first two public impeachment hearings, await the Nov. 20 testimony of Ambassador Gordon Sondland, who may be the only witness with firsthand knowledge of whether President Trump abused power by seeking Ukraine’s help investigating a political rival.

Sondland, the U.S. ambassador to the European Union, has already provided hours of closed-door testimony. But his value as a witness skyrocketed thanks to a new revelation provided by a State Department employee who said he overheard Sondland talking to Trump on a cellphone last summer.