Democrats on the House Judiciary Committee have armed their chairman with a tentative subpoena for Matthew Whitaker ahead of a Friday hearing, but the acting attorney general -- who previously had agreed to testify -- warned in response that he won't show up unless lawmakers drop the threat.

"Weeks ago, in good faith, I voluntarily agreed to appear and testify on February 8 before the House Judiciary Committee," Whitaker wrote in a letter to Committee Chairman Jerrold Nadler Thursday afternoon. "Unfortunately, the Committee now has deviated from historic practice and protocol and taken the unnecessary and premature step of authorizing a subpoena to me, the Acting Attorney General, even though I had agreed to voluntarily appear."

TOP DEM NADLER THREATENS TO SUBPOENA ACTING AG WHITAKER, DESPITE HIS AGREEMENT TO TESTIFY

He issued the warning as the committee voted Thursday to give Nadler, D-N.Y., the authority to subpoena Whitaker if he balks. But this set off a chain reaction, prompting Whitaker to threaten to do just that.

"Based upon today’s action, it is apparent that the Committee’s true intention is not to discuss the great work of the Department of Justice, but to create a public spectacle. Political theater is not the purpose of an oversight hearing, and I will not allow that to be the case," Whitaker wrote. "Consistent with longstanding practice, I remain willing to appear to testify tomorrow, provided that the Chairman assures me that the Committee will not issue a subpoena today or tomorrow, and that the Committee will engage in good faith negotiations before taking such a step down the road.”

Nadler originally invited Whitaker to appear before the committee for an open hearing scheduled for Friday. A Justice Department spokesperson told Fox News earlier this week that Whitaker had, in fact, accepted Nadler’s invitation to testify in public.

But, while making clear he does not want to have to compel Whitaker’s testimony, Nadler said “a series of troubling events” suggested he should be prepared, just in case he doesn’t show up for his hearing.

“In an abundance of caution—to ensure that Mr. Whitaker both appears in the hearing room on Friday morning and answers our question cleanly—I have asked the Committee to authorize me to issue a subpoena to compel his testimony,” Nadler said on Tuesday. “To be clear, I hope never to use this subpoena.”

But the top Republican on the committee, Ranking Member Doug Collins, R-Ga., blasted Nadler's decision.

WHITAKER: MUELLER PROBE IS 'CLOSE TO BEING COMPLETED'

“A subpoena is a powerful and coercive tool. And it is a tool that should be used as a last resort, especially when it implicates the balance of powers that exist between Congress and another branch of government,” Collins said earlier this week. “You asked the Acting Attorney General to come testify. He agreed. Both parties engaged in a back and forth regarding scheduling. You and the Acting Attorney General agreed upon tomorrow’s testimony. That is exactly how the process should work.”

Collins added: “A subpoena should only follow the breakdown of an accommodation process, and as a last resort against persons seeking to frustrate the legitimate oversight of this committee. There has been no breakdown here.”

Collins said the subpoena for Whitaker was “nothing short of political theater,” but also said that should Whitaker break his promise to appear before the committee voluntarily, then he would support a subpoena.

The hearing comes as the Senate is close to confirming President Trump’s nominee for attorney general, William Barr. The president fired his first attorney general, Jeff Sessions, the day after the 2018 midterm elections. Prior to Whitaker’s appointment as acting attorney general, Deputy Attorney General Rod Rosenstein oversaw the Russia investigation. The Senate Judiciary Committee on Thursday voted, on party lines, to advance Barr's nomination to the full Senate for confirmation.

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Nadler has criticized Whitaker for not recusing himself from the Mueller probe, as Sessions did due to his involvement with the Trump campaign in 2016, as Whitaker has made comments criticizing the investigation.

The hearing will be the committee’s first major oversight hearing looking at the Justice Department of this Congress. Whitaker told reporters last week that Mueller’s probe was “close to being completed,” the first official sign that the investigation may be nearing an end. His comments were a departure for the Justice Department, which rarely comments on the status of investigations. Whitaker, though, said he had been “fully briefed” on the probe.

Meanwhile, senior Justice Department officials told Fox News that Whitaker had been engaged in “robust” preparations for his appearance before the committee for weeks, including several mock hearings and briefings from all components within the Justice Department.

Fox News' Jake Gibson and the Associated Press contributed to this report.