William Taylor, the top U.S. diplomat in Ukraine, will leave his position on Jan. 1, according to a person familiar with his plans.

Taylor plans to turn over his responsibilities as chargé d'affaires at the U.S. Embassy in Kyiv to the current deputy chief of mission, Kristina Kvien, on Jan. 1 and leave Ukraine’s capital the following day, according to the source.

Taylor’s departure was first reported by NBC News. Under the Federal Vacancies Reform Act, Taylor could remain at the post in Kyiv until Jan. 8, and his contract at the State Department extends longer than that, the source familiar with his plans said.

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Sen. Bob Menendez Robert (Bob) MenendezWatchdog confirms State Dept. canceled award for journalist who criticized Trump Kasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report MORE (N.J.), the top Democrat on the Senate Foreign Relations Committee, wrote to Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE on Tuesday raising concerns about Taylor’s reported exit and suggesting he was “unceremoniously recalling” Taylor from Ukraine.

“I am extremely concerned that this suspect decision furthers the president’s inappropriate and unacceptable linking of U.S. policy to Ukraine to his personal and political benefit, and potentially your own,” Menendez said.

Taylor was a key witness in the House impeachment inquiry into President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE’s dealings with Ukraine. Taylor testified privately and publicly before the House Intelligence Committee in connection with the investigation, describing a rogue foreign policy channel spearheaded by Trump’s personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE.

Taylor alleged an effort by Trump administration officials to condition a White House meeting and nearly $400 million in security assistance on Ukraine launching investigations into Burisma, an energy company that employed Hunter Biden on its board, and 2016 election interference.

Trump raised the investigations on a July 25 call with Ukrainian President Volodymyr Zelensky that is at the heart of the impeachment inquiry. Trump has defended the phone call as “perfect,” denying any wrongdoing and maintaining there was no quid pro quo in his dealings with Ukraine.

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Trump lashed out at Taylor during the impeachment proceedings, labeling him a “Never Trumper” and tacitly criticizing Pompeo for tapping him for the role.

Pompeo persuaded Taylor, a veteran diplomat last appointed to a post by George W. Bush, to come out of retirement to head the U.S. Embassy in Ukraine earlier this year after Trump recalled Marie Yovanovitch Marie YovanovitchGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Strzok: Trump behaving like an authoritarian Powell backs Biden at convention as Democrats rip Trump on security MORE as U.S. ambassador to Ukraine.

Yovanovitch, who also testified in connection with the impeachment proceedings, was told Trump lost confidence in her after she endured a smear campaign from Giuliani and his allies. The Trump administration has not selected a permanent replacement for the role, which requires Senate confirmation.

It is unclear what Taylor’s professional plans are after he returns from Kyiv. The State Department did not return a request for information following reports he would leave the position.

The Democratic-controlled House is poised to vote Wednesday on two articles of impeachment against Trump, accusing him of abuse of power and obstruction of Congress. Trump issued a scathing letter to Speaker Nancy Pelosi Nancy PelosiPelosi: Trump Supreme Court pick 'threatens' Affordable Care Act Sunday shows preview: Lawmakers prepare for SCOTUS confirmation hearings before election Will Democrats attempt to pack the Supreme Court again? MORE (D-Calif.) on Tuesday accusing House Democrats of “perversion of justice and abuse of power” and urging her to halt the proceedings.

—Updated at 5:46 p.m.