In February, The New York Times reported that the president overruled several senior officials in order to reinstate the security clearance of Jared Kushner, his son-in-law and senior adviser, despite issues raised during his clearance process.

John F. Kelly, the president’s former chief of staff, wrote a contemporaneous internal memo about how he had been “ordered” to give Mr. Kushner the top-secret clearance. In her interviews with the House committee, Ms. Newbold said that Mr. Kelly and Joe Hagin, the former deputy chief of staff, had been attentive to the national security issues she had tried to raise.

Mr. Hagin resigned last June and Mr. Kelly in December. Mr. Trump has the legal authority to grant a clearance, but in most cases, the Personnel Security Office makes a determination about whether to grant one after the F.B.I. has conducted a background check.

Democrats on the committee are also demanding information from the White House about Ivanka Trump, the president’s eldest daughter and adviser, who was granted a security clearance shortly after Mr. Kushner got his. In an interview with ABC News in February, Ms. Trump insisted her father had no hand in either her clearance or her husband’s.

Ms. Newbold, who has a rare form of dwarfism, also accused Mr. Kline, the former director of personnel security, of retaliating against her when she repeatedly pointed out to him that actions he was taking, including overriding recommendations to issue clearances to two senior officials, were violating protocol. Ms. Newbold said that Mr. Kline oversaw a workplace where files — including extensive and sensitive background check documents — were not secured properly, and stopped the performing of credit checks for potential employees. She told the House committee that she had “never seen our office so ill-staffed and with such lack of experience.”

Last fall, Ms. Newbold filed a complaint with the Equal Employment Opportunity Commission, accusing Mr. Kline of moving office files to a shelf several feet above her, deliberately out of her reach, beginning in December 2017. That month, she hired Mr. Passman, her lawyer.

“As little as I am, I’m willing to fight and stand up for what I know is right, and they’ve always respected that about me,” Ms. Newbold told the House committee last week. “It’s humiliating to not be able to independently work and do the job that you need.”