A longtime career employee at the White House Office of Management and Budget is expected to break ranks and testify Saturday in the House Democrats’ impeachment inquiry, potentially filling in important details on the hold-up of military aid to Ukraine.

Mark Sandy would be the first OMB employee to testify in the inquiry, after OMB acting director Russell T. Vought and two other political appointees at the agency defied congressional subpoenas to appear. The White House has called the impeachment inquiry unconstitutional and ordered administration officials not to participate.

But unlike these other OMB officials, Sandy is a career employee, not one appointed by the president. He has worked at the agency off and on for over a decade, under presidents of both parties, climbing the ranks into his current role as deputy associate director for national security programs.