House Democrats are seeking to interview White House budget director Russell Vought on Oct. 25, according to a copy of the letter to the Office of Management and Budget​ obtained by ABC News, the latest sign that they are increasingly focused on the withholding of nearly $400 million in military aid to Ukraine as part of their impeachment investigation.

Vought, the acting director of the White House Office of Management and Budget, would be among the most senior administration officials called to appear before Congress in the Ukraine probe, though multiple sources told ABC News that the White House is likely to block their appearances before the committee, as they have vowed not to cooperate with the Democrats' investigation.

The White House and OMB did not immediately respond to requests for comment.

The committees investigating the matter have also sought interviews with several Pentagon officials, along with Michael Duffey, an associate director of national security programs at OMB, according to requests obtained by ABC News.

The issue of military aid is at the heart of the Democrats’ impeachment inquiry, which is focused on whether President Donald Trump withheld aid from Ukraine to pressure the country to investigate former Vice President Joe Biden’s family and a conspiracy theory related to the 2016 presidential election.

Democrats subpoenaed OMB and the Pentagon last week for documents related to the aid, along with the events surrounding Trump’s request to acting Chief of Staff Mick Mulvaney to freeze that aid in early July, before his phone call with Ukrainian President Volodymyr Zelenskiy. The deadline for those requests for documents is Tuesday, according to the subpoenas.

The two leaders discussed U.S. military aid to Ukraine on the call, according to a rough transcript released by the White House. Trump also appeared to pressure Zelenskiy to work with the Justice Department and his personal lawyer to conduct investigations linked to the 2016 election and Biden, a potential 2020 rival.

At a news conference at the United Nations General Assembly last month, Trump said he decided to withhold the aid because of concerns about corruption, later adding that he wanted other European allies to pay for military aid to Ukraine as well.

The Pentagon announced plans in June to provide Ukraine with $250 million in security cooperation funds, after the administration had told Congress it was releasing the aid February and May. The money was unfrozen on Sept. 11, after lawmakers in the House and Senate raised questions about the delay.

“We approved the money. The president signed it and we just assumed it was going out,” House Armed Services Committee Chairman Adam Smith told NPR earlier this month. “Then we started to hear from a variety of people that it was not going out.

Defense Secretary Mark Esper, in an interview with CBS News’ “Face the Nation” on Sunday, said the Pentagon would “do everything we can to comply” with the Democrats’ subpoena.