The White House is preparing to reassign a key witness from President Trump's impeachment trial: the National Security Council director and Army lieutenant colonel who reported the president's July 25 phone call to Ukraine's new leader.

Lt. Col. Alexander Vindman, a Ukraine expert who leads European affairs at the NSC, is expected to be rotated out of his current post as early as Friday.

Vindman's report to the White House counsel of Trump's phone call with Ukrainian President Volodymyr Zelensky, during which the president pressed for an investigation into former Vice President Joe Biden and his son Hunter Biden, is one of two accounts that led to the president's impeachment in the House. The second alarm was raised by a whistleblower, believed to be Eric Ciaramella, a CIA analyst who oversaw the NSC's Ukraine portfolio under President Barack Obama and briefly under Trump. A partial transcript of the phone call was later shared by the White House.

Testifying in November, Vindman said the president used "inappropriate" pressure on Zelensky. The move to oust Vindman is said to have been under consideration since "at least last week," Bloomberg reports.

Lower-level officials in nonpolitical positions have typically been safe from retaliatory firings, and doing so is likely to draw criticism from Congress. According to two people familiar with the plan who spoke to Bloomberg, "The White House intends to portray any house-cleaning as part of a downsizing of the NSC staff, not retaliation."

Vindman told senior NSC colleagues that he expected to leave his post as early as the end of the month, according to people familiar with his decision who spoke to the Washington Post. But Trump is eager to draw attention to Vindman and what Trump perceives as his perfidy.

Vindman's next position is likely to be at the Pentagon under Defense Secretary Mark Esper, who said this summer Vindman would not face retribution for his actions.