According to Cummings, the whistleblower said during the March interview that the list of approximately 25 individuals whose denials were overruled "had a wide range of serious disqualifying issues involving foreign influence, conflicts of interest, concerning personal conduct, financial problems, drug use, and criminal conduct."

That whistleblower, longtime White House security specialist Tricia Newbold, spoke with Republicans and Democrats on the committee in a private interview on March 23, according to Oversight Chairman Elijah Cummings , D-Md.

More than two dozen people were granted security clearances or access to national security secrets by President Donald Trump's White House — despite recommendations from officials that they be denied, a whistleblower told the House Oversight and Government Reform Committee.

"She has come forward at great personal risk to warn Congress — and the nation — about the grave security risks she has been witnessing first-hand over the past two years," Cummings said in a letter sent Monday to White House counsel Pat Cipollone.

Cummings told Cipollone that his committee plans to begin authorizing subpoenas beginning at a business meeting Tuesday.

The White House did not immediately respond to CNBC's request for comment on Cummings' letters.

Newbold, who has worked in the Executive Office of the President for 18 years, received a two-week suspension without pay in January for defying her supervisor, Carl Kline, NBC News reported at the time. That action came just days after reports surfaced that Kline had approved a top secret clearance for Trump's son-in-law, Jared Kushner, over the objections of security officials.

Cummings' first subpoena will be aimed at getting a deposition of Kline, Cummings told Cipollone. Kline, who now works at the Defense Department, has not replied to multiple requests for an interview from the committee, Cummings claimed.

Newbold "agreed to identify herself publicly at this time because she strongly believes that Congress must intervene immediately to safeguard our national security," Cummings wrote.

He quotes Newbold as saying: "I would not be doing a service to myself, my country, or my children if I sat back knowing that the issues that we have could impact national security."

"I feel that right now this is my last hope to really bring the integrity back into our office," she added, according to Cummings.

Republicans on the Oversight Committee were quick to push back against the Cummings letter. On Monday afternoon, Rep. Jim Jordan, R-Ohio, the top Republican on the committee, released a nine page memo highlighting elements of Newbold's testimony which he claimed had been glossed over or mischaracterized by Cummings and his staff.

The memo authored by GOP committee staff accused Democrats of attempting to, "manufacture a misleading narrative that the Trump White House is reckless with our national security."

In one instance, committee staff highlighted the fact that of the 25 cases in which Newbold recommended against a security clearance only to be overruled by Kline later on, "only 4-5 of her unfavorable 25 adjudications were for 'very serious reasons.'"

"If Kline overturned only—at most—five clearance adjudications with very serious concerns," they wrote, "Ms. Newbold's concerns seem overblown."

They also pointed out that Newbold's testimony "focused primarily on problems with her supervisor," an apparent attempt to portray Newbold's complaint as more of a workplace issue than a national security issue.

Nonetheless, they acknowledged that Newbold's "primary concern" with Kline was not a personnel issue, per se, but was instead the fact that Kline had overturned 25 unfavorable security clearance determinations that Newbold had made.