The House impeachment inquiry is exposing new details about unease in the State Department and White House about President Donald Trump's actions toward Ukraine and those of his personal lawyer Rudy Giuliani.
One witness said it appeared "three amigos" tied to the White House had taken over foreign policy.
Another quoted national security adviser John Bolton as calling Giuliani a "hand grenade" for his back-channel efforts to get Ukraine to investigate Trump's Democratic rival Joe Biden and Biden's son Hunter.
On Wednesday, a former aide to Secretary of State Mike Pompeo is to speak to the House impeachment panels behind closed doors.
Michael McKinley, who resigned last week, is a career foreign service officer and was Pompeo's de facto chief of staff.
He is expected to discuss concerns held by career State Department officials about the treatment of the US ambassador to Ukraine, Marie Yovanovitch, and others who worked on the Ukraine portfolio, according to a person familiar with his testimony.
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A Latin America specialist, McKinley wasn't directly involved in Ukraine policy, but as a senior adviser to Pompeo was generally aware of the situation, the person said.
McKinley expects to talks about demoralisation in the ranks of career foreign service officers and what many have lamented as the politicization of the once-apolitical bureaucracy, according to the person who spoke on condition of anonymity.
The 37-year veteran of the diplomatic corps was known to be unhappy with the state of affairs although his farewell note to colleagues mentioned nothing about the reason for his departure other than it was a "personal decision."
Another witness, former White House aide Fiona Hill, testified Monday that Bolton, then national security adviser, was so alarmed by Giuliani's back-channel activities in Ukraine that he described him as a "hand grenade who is going to blow everybody up."