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 holdup 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.

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 to his current role as deputy associate director for national security programs.

“If he is subpoenaed, he will appear,” Sandy’s lawyer, Barbara “Biz” Van Gelder, said Thursday evening.

Sandy is expected to testify during a closed-door deposition, which is not open to the public. Typically, witnesses in the impeachment inquiry have been served with subpoenas immediately before their depositions are scheduled to begin, an approach Democrats say is designed to give them cover against an administration that has ordered officials not to comply with the inquiry.

Van Gelder declined further comment.

The Washington Post's Shane Harris, Paul Kane and Amber Phillips break down what happens next in the effort to impeach President Trump. (Zach Purser Brown/The Washington Post)

Until now, OMB has served as a bulwark for President Trump against the impeachment inquiry, since top officials have refused to testify. A senior administration official, speaking on the condition of anonymity to discuss the ongoing inquiry, discounted the importance of Sandy’s testimony and criticized House Speaker Nancy Pelosi (D-Calif.) and Intelligence Committee Chairman Adam B. Schiff (D-Calif.) for calling him to testify.

“Democrats are going to be sorely disappointed when their latest false narrative isn’t confirmed,” the official said. “With nothing to show after three years trying to impeach the President, Speaker Pelosi and Rep. Schiff have resorted to threatening dedicated civil servants with subpoenas and depositions without the ability to even have agency counsel present.”

Sandy could provide insight into the process by which some $400 million in military and security aid to Ukraine was held up over the summer. He was among the career staffers who raised questions about the holdup on the aid, people familiar with the matter said, and his role gave him responsibility for signing the documents required to hold it up. The people spoke on the condition of anonymity to discuss internal deliberations.

Sandy’s signature appears on at least one of these so-called apportionment letters in July that prevented the money from going to Ukraine, according to copies of the documents discussed during an earlier deposition in the impeachment inquiry, a transcript of which was made public. But after that, the process for approving or denying such funds was taken over by a political appointee at OMB, Michael Duffey, who defied a congressional subpoena to testify earlier this month. The money had already been approved by Congress.