House Judiciary Committee Chairman Jerrold Nadler is threatening to subpoena Acting Attorney General Matthew Whitaker, despite the Justice Department leader already agreeing to appear before the panel.

Nadler, D-N.Y., originally invited Whitaker to appear before the committee for an open hearing scheduled for Friday. A Justice Department spokesperson told Fox News on Tuesday that Whitaker had, in fact, accepted Nadler’s invitation to testify in public.

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But, while making clear he does not want to have to compel Whitaker's testimony, Nadler said he would hold a vote on Thursday to authorize a subpoena -- just in case he doesn’t show up for his hearing the following day.

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

Nadler explained that Democrats on the committee “weeks ago” gave Whitaker a list of questions they hoped he’d answer regarding his communications with the White House and his decision not to recuse himself from oversight of Special Counsel Robert Mueller’s investigation.

“If he appears on time and ready to answer those questions, the subpoena will be entirely unnecessary,” Nadler said.

But the top Republican on the committee, Rep. Doug Collins, R-Ga., criticized the chairman for “setting a dangerous precedent.”

“When did we start subpoenaing witnesses who come in voluntarily?” Collins told Fox News on Tuesday. “The majority had enough faith in its witnesses last week not to subpoena them. The key difference today is simply that this witness is part of the Trump Administration—and now we’re setting a dangerous precedent.”

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He added: “The message to witnesses here is, if you make the time and effort to appear of your own accord, Democrats are going to subpoena you anyway.”

Whitaker’s expected testimony 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.

But 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.

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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.

Fox News' Sally Persons and The Associated Press contributed to this report.