The White House late Tuesday afternoon informed House Speaker Nancy Pelosi and House Democratic leaders in a scathing eight-page letter that President Donald Trump and his administration is refusing to participate in the "illegitimate and unconstitutional" impeachment inquiry.

"President Trump and his administration reject your baseless, unconstitutional efforts to overturn the democratic process," the letter, written by White House counsel Pat Cipollone, says. "Your unprecedented actions have left the president with no choice. In order to fulfill his duties to the American people, the Constitution, the Executive Branch, and all future occupants of the Office of the Presidency, President Trump and his administration cannot participate in your partisan and unconstitutional inquiry under these circumstances."

Administration officials called the letter the most historic the White House has sent, reports Fox News, and comes after numerous subpoenas coming already as the House inquiry builds.

"The president has a country to lead," the letter states. "The American people elected him to do this job, and he remains focused on fulfilling his promises to the American people."

The White House also pointed out that a formal vote was not held for the inquiry, and that it is being conducted without a majority of the House voting in approval of it. Other impeachment proceedings for Presidents Andrew Johnson, Richard Nixon, and Bill Clinton all started with votes for inquiries, rather than an announcement such as Pelosi had made.

"Information has recently come to light that the whistleblower had contact with [House Intelligence Committee] Chairman [Adam] Schiff's office before filing the complaint," the letter said, but Schiff denied it.

"The American people understand that Chairman Schiff cannot covertly assist with the submission of a complaint, mislead the public about his involvement, read a counterfeit version of the call to the American people, and then pretend to sit in judgment as a neutral 'investigator,'" the letter said.

In addition, even after the call's transcript was released, Schiff chose to present a "made-up" transcript at a hearing, which undermined confidence in the fairness of inquiries coming before his committee.

The letter also claimed several violations of Trump's due-process rights, including not allowing presidential or State Department attorneys to be present at impeachment inquiry proceedings.

The letter also said that disclosure of all evidence favorable to the president has not been allowed, nor was Trump allowed the right to call witnesses or see evidence.

House intelligence committee Chairman Adam Schiff tweeted in response that Trump's refusal to cooperate with the inquiry signals an attitude that "the president is above the law."

"The Constitution says otherwise," he asserted.

House Speaker Nancy Pelosi has insisted the House is well within its rules to conduct oversight of the executive branch under the Constitution regardless of a formal impeachment inquiry vote.

"Mr. President, you are not above the law," Pelosi said in a statement Tuesday night. "You will be held accountable."

The Constitution states the House has the sole power of impeachment, and that the Senate has the sole power to conduct impeachment trials. It specifies that a president can be removed from office for "treason, bribery, or other high crimes and misdemeanors," if supported by a two-thirds Senate vote. But it offers little guidance beyond that on proceedings.

The White House letter marks the beginning of a new all-out strategy to counter the impeachment threat to Trump. Aides have been honing their approach after two weeks of what allies have described as a listless and unfocused response to the probe.

The president himself is sticking with the same rhetoric he has used for more than a year.

"People understand that it's a fraud. It's a scam. It's a witch hunt," he said on Monday. "I think it makes it harder to do my job. But I do my job, and I do it better than anybody has done it for the first two and half years."

Early Tuesday, Trump escalated his fight with Congress by blocking Gordon Sondland, the U.S. European Union ambassador, from testifying behind closed doors about the president's dealings with Ukraine.

Sondland's attorney, Robert Luskin, said his client was "profoundly disappointed" that he wouldn't be able to testify. And Schiff said Sondland's no-show was "yet additional strong evidence" of obstruction of Congress by Trump and Secretary of State Mike Pompeo that will only strengthen a possible impeachment case.

The House followed up Tuesday afternoon with subpoenas for Sondland's testimony and records.

Trump is also bulking up his legal team.

Former Republican Rep. Trey Gowdy is being brought on as outside counsel, according to an administration official. Gowdy, who did not seek reelection last year, led a congressional investigation of former presidential candidate Hilary Clinton and the terrorist attacks in Benghazi, Libya.

The whistleblower's complaint and text messages released by another envoy portray U.S. Ambassador Sondland as a potentially important witness in allegations that the Republican president sought to dig up dirt on Democratic rival Joe Biden in Ukraine and other countries in the name of foreign policy.

Pelosi said thwarting the witness testimony on Tuesday was an "abuse of power" in itself by the president.

The White House letter to Pelosi, Schiff and other House committee chairmen, though asserting a legal argument that Trump and other officials cannot cooperate, would not be likely to win respect in court, said Stephen Vladeck, a law professor at the University of Texas.

"This letter reads to me much more like a press release prepared by the press secretary than an analysis by the White House counsel," he said.

The White House is claiming that Trump's constitutional rights to cross-examine witnesses and review all evidence in impeachment proceedings extend even to House investigations, not just a potential Senate trial. It also is calling on Democrats to grant Republicans in the House subpoena power to seek evidence in the president's defense.

Elsewhere in Washington, a federal judge heard arguments Tuesday in a separate case on whether the House has actually undertaken a formal impeachment inquiry despite not having taken a vote and whether the inquiry can be characterized, under the law, as a "judicial proceeding."

That distinction matters because while grand jury testimony is ordinarily secret, one exception authorizes a judge to disclose it in connection with a judicial proceeding. House Democrats are seeking grand jury testimony from special counsel Robert Mueller's Russia investigation as they conduct their impeachment inquiry.

"The House under the Constitution sets its own rules, and the House has sole power over impeachment," Douglas Letter, a lawyer for the House Judiciary Committee, told the court.

The Associated Press contributed to this report.