Rep. Jerrold Nadler (D-N.Y.) said Sunday that the House Judiciary Committee will subpoena acting Attorney General Matt Whitaker Matthew G WhitakerEx-federal prosecutor: 'Thank God' Whitaker is gone, Barr will bring 'integrity' back to DOJ GOP pollster says Dems are relitigating 2016 election with investigations of Trump Former senior FBI official calls Whitaker hearing ‘disgraceful’ MORE in January if he won't voluntarily testify in front of the committee.

Nadler, the ranking member and likely next chairman of the Judiciary Committee, said on CNN's "State of the Union" that the committee will hold hearings on Trump's appointment of Whitaker when the new Congress takes office.

Nadler added that the "very first witness" the committee will interview will be Whitaker.

"The president’s dismissal of Attorney General Sessions and his appointment of Whitaker, who’s a complete political lackey, is a real threat to the integrity of that investigation," he said of special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's ongoing probe into Russia's election interference.

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"[The Mueller investigation] is of utmost importance in making sure that we adhere to the rule of law and that the administration is held accountable. And we will certainly hold hearings on that. Our very first witness after Jan. 3, we will subpoena — or we will summon and if necessary subpoena Mr. Whitaker," Nadler added.

Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE resigned last week from his post as attorney general at the request of President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE, who then appointed Whitaker as acting attorney general.

The appointment has been harshly criticized by Democrats, who have expressed concerns over Whitaker's history of criticizing Mueller's investigation.

Whitaker previously said that the Mueller investigation has "gone too far" and suggested that a replacement for Sessions should slash funding for Mueller's investigation in an effort to hinder it.

Nadler said Sunday on "State of the Union" that the Judiciary Committee plans to question Whitaker about each of his past comments regarding the investigation he now oversees.