Acting Attorney General Matthew Whitaker said he will bail on his scheduled testimony in the House Judiciary Committee on Friday unless Democrats assure him they will not file the subpoena they voted to authorize along party lines on Thursday.

At the heart of the disagreement — and why Democrats green-lighted the preemptive subpoena-in-reserve in the first place — is Democrats’ wariness that Whitaker will avoid answering certain questions about his communications with President Donald Trump about the special counsel investigation of Robert S. Mueller III and other hot-button issues by citing, without effectively asserting, “executive privilege.”

Democrats want to avoid what happened at a hearing in 2017 with former Attorney General Jeff Sessions. At that voluntary hearing, Sessions declined to answer questions on conversations between top Justice Department and White House officials because the president might want to invoke executive privilege.

If Trump is going to assert executive privilege, Democrats want his administration officials to declare it under subpoena, opening up the possibility for a court challenge.

Because Whitaker is attending his Friday hearing voluntarily — for now — he, like Sessions, does not need to explicitly invoke executive privilege to avoid answering some questions. That, of course, changes if Chairman Jerrold Nadler slaps him with a subpoena.

In a statement on Thursday, Whitaker claimed that the committee “has deviated from historic practice and protocol and taken the unnecessary and premature step of authorizing a subpoena to the me… even though I had agreed to voluntarily appear.”

Republicans on the committee sided with the acting AG.

“Chairman Nadler and Democrats overplayed their hand,” ranking member Doug Collins of Georgia said. “In a quest to score political points against the president, they authorized a preemptive subpoena, treating a voluntary witness as hostile.”

In a Jan. 22 letter, Nadler outlined a series of questions that he plans to ask Whitaker at the hearing that “may conceivably implicate executive privilege” since they probe details of Whitaker’s conversations with the president and other White House advisers.

Nadler advised Whitaker to consult with the White House counsel on whether the president would invoke executive privilege to preclude Whitaker from answering those and other lines of inquiry, and to notify the committee of the White House’s decisions.

Whitaker did not meet the deadline to inform the committee within 48 hours of the hearing whether he would cite executive privilege, so Democrats went ahead with their plans to authorize, but not file, the preemptive subpoena.

On Thursday, after Democrats authorized the subpoena, Whitaker responded to Nadler in a letter saying that Nadler had breached their agreement earlier in January for Whitaker to appear before the committee voluntarily.

Nadler has said he does not want to file the subpoena and would only do so as a last resort if Whitaker provides dodgy testimony.

“I hope not to have to use the subpoena,” Nadler said Thursday. “Unfortunately a series of troubling events over the last few months suggest that we should be prepared.”

Watch: Trump Says He Doesn’t Know Matt Whitaker