The letter, published on Tuesday evening, condemned the impeachment initiative in the harshest terms yet, arguing it deprived President Trump of “constitutionally mandated due process,” and that the inquiry lacked legal legitimacy, as it was never authorized by a House vote.

“You have conducted your proceedings in secret. You have violated civil liberties and the separation of powers,” the letter continued “All of this violates the Constitution, the rule of law, and every past precedent.” [emphasis in original]

The White House accused Democrats of using impeachment as a tool to not only “undo the democratic results” of the previous election, but to “influence” the upcoming contest as well, citing the words of Congressman Al Green (D-Texas), who in May expressed concerns that “if we don't impeach the President, he will get reelected.”

The letter also notes that ranking Republican committee members had not been granted the same subpoena powers as the Democratic chairmen leading the impeachment process – as they were during previous inquiries – slamming the process as unfair and “one-sided.”

Earlier on Monday, the Democratic committee chairs issued a subpoena to compel the testimony of EU Ambassador Gordon Sondland, a key figure in the inquiry, after the White House signaled that it would block his appearance before Congress. The Trump administration appears to be doubling down on that move, arguing in the letter that it will simply not comply with future subpoenas.

Going along with the inquiry under its “current unconstitutional posture” would “inflict lasting institutional harm on the Executive Branch and lasting damage to the separation of powers,” the letter said, adding that Democrats have “left the president no choice” but to refuse to cooperate.

The missive is the White House’s latest response to intensifying impeachment efforts spearheaded by House Democrats, who launched the proceedings late last month accusing Trump of pressuring the President of Ukraine to probe into the activities of former Vice President Joe Biden and his son in the country.

Several Democratic opponents shot back at the document, some denouncing the move as an act of obstruction.

“The White House letter is only the latest attempt to cover up his betrayal of our democracy, and to insist that the President is above the law,” said House Speaker Nancy Pelosi in a statement on Tuesday, adding the president had “normalize[d] lawlessness.”