Ms. Newbold, who has worked in the White House for 18 years under both Republican and Democratic administrations, said she chose to speak to the committee after attempts to raise concerns with her superiors and the White House counsel went nowhere, according to the committee staff’s account.

“I feel that right now, this is my last hope to really bring the integrity back into our office,” she said, according to a summary of her March 23 interview with the committee’s staff that was distributed on Monday.

White House officials have been concerned for weeks that Ms. Newbold would either speak publicly or share information that she had gleaned about how security clearances had been handled during the first half of Mr. Trump’s term. Her statements to the Oversight Committee are likely to increase pressure on the White House to address lingering questions about its general practices around keeping the nation’s secrets and several high-profile cases.

Representative Elijah E. Cummings, Democrat of Maryland and the chairman of the committee, included information provided by Ms. Newbold in a letter to Pat A. Cipollone, the White House counsel, on Monday, again demanding that the White House turn over files connected to the security clearance process and make administration personnel available for interviews.

Mr. Cummings said he was prepared to authorize subpoenas as soon as Tuesday to try to compel the White House to comply with an investigation into whether national secrets were at risk — an escalation that could force Mr. Cipollone either to reach an accommodation with Congress or fight in court.

“I understand that the president has the right to grant these security clearances, but I have the duty and Congress has a duty to be a check on that system,” Mr. Cummings told reporters. “If the top secrets of our country are being seen by eyes that should not see them, we ought to all be concerned.”