Michael McKinley, a former senior adviser to Secretary of State Mike Pompeo, is set to deliver damning testimony to Congress about the ouster of former Ukrainian ambassador Marie Yovanovitch.

The Washington Post reports that McKinley will tell investigators on Wednesday that he resigned from the State Department because he was disgusted by its unwillingness to stick up for Yovanovitch, who was removed from her post after pressure from both President Donald Trump and personal attorney Rudy Giuliani.

“The unwillingness of State Department leadership to defend Yovanovitch or interfere with an obviously partisan effort to intervene in our relationship with Ukraine for the political benefit of the president was too much for him,” explained one source who is described by the Post as being “familiar” with McKinley’s planned testimony.

The Post’s source claims that McKinley will not directly criticize Pompeo in his prepared remarks, although he will discuss his concerns about ways the State Department has become politicized under the Trump administration as a whole.

McKinley resigned last Friday, although the Post says that he had told Pompeo more than a week earlier about his plans to step down. The former State Department official has worked as an American diplomat for the past 37 years.