Watch live: Gordon Sondland testifies at Trump impeachment hearing

Gordon Sondland, U.S. ambassador to the European Union, has emerged as a pivotal link between the president and a shadow foreign policy led by Rudolph W. Giuliani, who was urging the Ukrainians to conduct investigations into former Vice President Joe Biden and his son as military aid to Ukraine was being held up.

Sondland will be the first of three witnesses to testify Wednesday. In the afternoon, lawmakers will hear testimony from Laura Cooper, a deputy assistant secretary at the Department of Defense who specialized in Ukraine issues, and David Hale, the undersecretary of State for political affairs who was stationed in Kyiv.