Charge d'affaires at the U.S. Embassy in Ukraine William Taylor speaks during a press-conference about US-Ukrainian relations, in Kiev, Ukraine, on 27 July 2019.

Acting U.S. ambassador to Ukraine Bill Taylor left Kyiv en route to Washington, D.C., NBC News reported Wednesday, two days after House Democratic leaders asked him to testify as part of their impeachment inquiry into President Donald Trump.

House committee chairmen Reps. Adam Schiff, Eliot Engel and Elijah Cummings requested that Taylor appear at the Capitol for a deposition on Oct. 22, according to a letter sent Monday by the Democratic leaders.

Taylor is a key witness for the lawmakers leading the impeachment inquiry into Trump's desire to have Ukraine "look into" unsubstantiated corruption allegations against former Vice President Joe Biden — Trump's possible 2020 election opponent — and his son Hunter.

Taylor's possible cooperation comes after the White House took a defiant stance against the impeachment inquiry, calling the proceedings "baseless, unconstitutional efforts to overturn the democratic process" and vowing not to cooperate.

A lawyer for Taylor told CNBC that he was reaching out to his client for direction on "how much, if anything, he wishes us to say at this point."

Taylor emerged as a significant figure in the impeachment probe after former U.S. special envoy to Ukraine Kurt Volker provided copies of text messages to Congress as part of his deposition before the House Intelligence, Oversight and Foreign Affairs committees.

Those texts included Taylor's exchanges with U.S. ambassador to the European Union Gordon Sondland, who was scheduled to testify before the Democratic panels Thursday.

In a Sept. 9 exchange, Bill Taylor, a senior U.S. diplomat in Ukraine, told Sondland: "I think it's crazy to withhold security assistance for help with a political campaign."

Sondland responded: "Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign."

Click here to read those texts.

Sondland intends to tell Congress that his position in that text message came after a phone call with Trump, who relayed that denial to him directly, according to The Washington Post.