"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".
Nadler said that authorizing a subpoena for Whitaker was necessary because Whitaker failed to tell the committee whether the Trump administration would invoke privilege with respect to a series of questions they intend to ask about Whitaker's conversations with the White House about Mueller's probe and his decision not to recuse himself from the matter.
The questions Democrats want to ask Whitaker - outlined in a January 22 letter to which the AG did not respond until Thursday - fall under "a category of communications that Administrations of both parties have viewed as raising confidentiality interests that are vital to a President's ability to discharge the responsibilities of his office", Whitaker wrote in his letter.
House Judiciary Committee Chairman Rep. Jerrold Nadler D-NY, speaks during a House Judiciary Committee debate to subpoena Acting Attorney General Matthew Whitaker, on Capitol Hill in Washington, Thursday, Feb. 7, 2019.
Democrats approved giving Nadler the authority to subpoena Whitaker over the objection of Republicans earlier on Thursday, passing the resolution on a party-line vote, 23-13. Whitaker responded by saying he won't appear unless the committee dropped its subpoena threat, which he derided as an act of "political theater".
He also has declined to recuse himself from overseeing the investigation after career ethics officials at the department urged him to step aside to avoid the appearance of a conflict.
Nadler has stressed that when Whitaker appears before Congress, he expects him to answer all questions "without equivocation".More news: What you need to know about the State of the Union address
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The committee vote on the subpoena along partisan lines underscores the new political tensions around Mueller's work now that Democrats control the House of Representatives.
Nadler explained, "I hope not to have to use the subpoena".
The threat of a subpoena now hangs over Friday's hearing with Whitaker, in what is likely to be his only congressional testimony while leading the Justice Department, with William Barr on a path to be confirmed as the permanent attorney general by the Senate next week.
He criticised the committee for prematurely and unnecessarily authorising a subpoena for him even though he had agreed to appear. "Such unprecedented action breaches our prior agreement and circumvents the constitutionally required accommodation process".
Whitaker has been undertaking significant preparations ahead of the hearing, including conducting briefings with every the Justice Department component and participating in several mock hearings, according to the officials.
Whitaker added that "political theater is not the goal of an oversight hearing". He's also likely to be asked about his comment at an unrelated news conference last week that he believed the investigation, into potential ties between Russian Federation and the Trump campaign, was almost done.
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.