House Judiciary Committee Chairman Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.) is threatening to hold former White House counsel Don McGahn in contempt if he does not testify publicly on Tuesday before Congress.

“First thing we are going to do is hold McGahn in contempt,” Nadler told CNN on Monday night, noting that the committee intends to then enforce such citations for defying congressional subpoenas in court.

"You are dealing with a lawless president who is willing to go to any lengths to prevent testimony that might implicate him — that does implicate him," Nadler continued.

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Separately, Nadler also issued a letter to McGahn Monday night saying he expects McGahn to appear, despite 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's order.

"President Trump’s order — which seeks to block a former official from informing a coequal branch of government about his own misconduct — is unprecedented and, contrary to the letter received from your counsel this evening, does not excuse your obligation to appear before the Committee," Nadler writes.

Nadler's remarks come after McGahn’s attorney William Burck notified the chairman in a letter Monday evening that his client would comply with instructions from the White House to not testify before his committee. By refusing to testify, McGahn is defying a congressional subpoena compelling his appearance.

Nadler suggested the White House's stonewalling is pushing Democrats closer and closer to an impeachment inquiry, a move Democrats on his committee voiced support for after news McGahn would not appear before Congress broke.

"The recalcitrance of the president and his lawless behavior is making it more and more difficult to ignore all alternatives including impeachment," Nadler told CNN. "The president cannot be above the law, no more than anyone else can."

In his letter, the New York chairman laid out a series of arguments that pushed back against the White House and McGahn's decision not to testify. He said the president has not sought to assert executive privilege over his testimony and that they have not claimed that attorney-client privilege is an obstacle for McGahn to provide testimony.

Nadler also writes that the president has attacked his former lawyer's credibility by claiming that he never planned to fire 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, a point that references McGahn's testimony to the special counsel detailing a series of matters that were investigated as possible obstruction of justice, including Trump's attempts to fire Mueller during his 22-month long investigation.

In arguing against his testimony, the Justice Department (DOJ) issued an opinion that pointed to decades of precedent in claiming White House aides cannot be compelled to speak to Congress about their official duties.

In 15-page legal opinion letter, the DOJ's Office of Legal Counsel made the case that McGahn should not attend due to concerns about separation of powers and that McGahn is not “legally required” to testify about matters related to his work for Trump.

Republicans and other allies of the administration also claim Democrats are carrying out overly broad investigations that are designed to hurt or undermine the president heading into 2020.

Democrats, however, are not going to allow a star witness to dodge public testimony without a fight. McGahn is seen as a key witness for Democrats due to the extensive testimony he gave to the special counsel.

“The Committee has made clear that you risk serious consequences if you do not appear tomorrow,” Nadler concludes in his letter to McGahn. “Should you fail to do so, the Committee is prepared to use all enforcement mechanisms at its disposal.”