U.S. Ambassador to Ukraine Bill Taylor will step down from his post in January, Taylor announced today.

The ambassador was a key witness in House Democrats’ impeachment inquiry into President Donald Trump, and his testimony last month helped spur what will likely be just the third presidential impeachment in U.S. history. (RELATED: Impeachment Proceedings Usually Move Swiftly, But Democrats Are Playing It Slow)  

“The administration will nominate a permanent ambassador soon,” Taylor said, according to The New York Times.

Taylor became the acting ambassador to Ukraine in June, and his tenure is set to expire in January under the Federal Vacancies Act. Taylor previously served as U.S. ambassador to Ukraine from 2006-2009.

During both his closed door and public testimony last month, Taylor criticized Trump’s foreign policy and emphasized the importance of the U.S.-Ukraine alliance. (RELATED: The Tide Is Turning Against Democrats On Impeachment)

“The security assistance we provide is crucial to Ukraine’s defense and to the protection of the soldiers I met last week,” Taylor said last month. “It demonstrates to Ukrainians — and Russians — that we are Ukraine’s reliable strategic partner. It is clearly in our national interest to deter further Russian aggression.”

Democrats are expected to officially impeach the president this week, before Congress recesses for Christmas.