The career diplomat, who served as ambassador to Ukraine, became a key figure in the impeachment of President Trump, was forced out of her post in May.
On Oct. 26, Reeker told lawmakers that senior State Department officials blocked him from helping Marie Yovanovitch, the former U.S. ambassador to Ukraine who was the subject of a pressure campaign.
The diplomat will be the sole witness Friday, the second day of public testimony in the impeachment inquiry. State Department colleagues say she was removed after a slander campaign by Rudy Giuliani.
Democrats have called three career public servants to appear for the first public hearings in the Trump impeachment inquiry: William Taylor, George Kent and Marie Yovanovitch.
Marie Yovanovitch said she found out from Ukrainian officials that President Trump's personal lawyer, Rudy Giuliani, and his associates were targeting her and wanted a different person on the job.
Marie Yovanovitch was recalled from her post in May after criticism from some Republicans, including the president's eldest son, that she was not supportive of the administration's policies.
