As the impeachment hearings enter their second week - one of the most highly anticipated testimonies will be Wednesday’s appearance from Gordon Sondland, the US ambassador to the EU.

Unlike the other witnesses in the inquiry, Sondland is not a career diplomat versed in the protocols of the State Department or a foreign policy expert. He’s a Seattle businessman who gave a million dollars to Trump’s inauguration and later became Trump's pick for envoy to the European Union. But Sondland’s direct interactions with Trump, and testimony from other witnesses about his dealing with Ukraine, have put him front and center in the controversy, over whether Trump made aid to Ukraine contingent on opening an investigation into his political rival, Joe Biden.

Sondland was one of three officials - along with Kurt Volker, the former representative for Ukraine, and Energy secretary Rick Perry - to take the lead on American policy toward Ukraine after Trump abruptly removed the U.S. ambassador in Kiev, Marie Yovanovitch.

Alexander Vindman, a U.S. Army lieutenant colonel and Ukraine expert on the NSC, said he heard Sondland explicitly press Ukrainian officials to investigate Joe Biden and his son, according to a transcript of his private testimony.

And On Friday David Holmes, a U.S. embassy official in Kiev, told lawmakers in closed-door testimony, that he overheard a phone call between Trump and Sondland, in which the ambassador told the president his Ukrainian counterpart was ready to carry out the investigations. The phone call occurred on July 26, one day after the phone conversation between Trump and Zelenskiy.

The top US diplomat in Ukraine Bill Taylor discussed that call in his public hearing.

(SOUNDBITE) (ENGLISH) DIPLOMAT WILLIAM TAYLOR, SAYING:

“The member of my staff could hear President Trump on the phone asking Ambassador Sondland about ‘the investigations.’ Ambassador Sondland told President Trump that the Ukrainians were ready to move forward. Following the call with President Trump, the member of my staff asked Ambassador Sondland what President Trump thought about Ukraine. Ambassador Sondland responded that President Trump cares more about the investigations of [Joe] Biden, which Giuliani was pressing for.”

Sondland appeared in released texts saying there was no quid pro quo in withholding aid. But after testifying behind closed doors, he then had to revise his testimony to say that by September he had come to view a suspension of U.S. security aid as being held up as leverage to get Ukraine to commit.

Up until now – among the GOP's main defenses of Trump are that there was no quid pro quo, and that no witnesses has spoken directly to Trump about the withheld aid. Democrats hope Sondland could undercut those arguments..