A defiant White House declared war Tuesday against the impeachment investigation of Donald Trump, blasting the process as partisan, illegitimate and unconstitutional and saying neither the president nor his administration will cooperate.

In a fiery letter, the White House threw down the gauntlet to House Speaker Nancy Pelosi and the chairmen of three congressional panels leading the inquiry, challenging them to proceed with their effort to oust Trump despite zero future cooperation.

"Put simply, you seek to overturn the results of the 2016 election and deprive the American people of the president they have freely chosen," White House counsel Pat Cipollone wrote in an eight-page letter.

"Your inquiry lacks any legitimate constitutional foundation, any pretense of fairness, or even the most elementary due process protections,” he said of the probe, which is weighing whether Trump abused his office by seeking a corruption probe in Ukraine of his rival Joe Biden.

“President Trump cannot permit his administration to participate in this partisan inquiry under these circumstances."

Pelosi fired back, terming the letter “manifestly wrong" and describing it as "another unlawful attempt to hide the facts."

"Mr President, you are not above the law. You will be held accountable,” she said, warning that “continued efforts to hide the truth of the president's abuse of power from the American people will be regarded as further evidence of obstruction."

The stand-off set Washington on a path toward a constitutional crisis and into unknown territory, with the potential fora court showdown just as the nation gears up for the 2020 election.

The White House letter capped a day of explosive developments that included Trump barring a key witness, Ambassador to the European Union Gordon Sondland, from speaking to Congress - calling it a "totally compromised kangaroo court."

In response, Democrats slapped Sondland with a subpoena that "compels" him to appear on October 16.

Meanwhile, a Senate Republican-led investigation said in a report Tuesday that Russia indeed sought to help Trump in the 2016 election, contradicting persistent White House claims to the contrary.

The Senate Intelligence Committee's 2016 election investigation report said a sweeping social media manipulation campaign by the St. Petersburg-based Internet Research Agency (IRA) represented "overt support of Russia's favored candidate in the US presidential election."

"The committee found that the IRA sought to influence the 2016 US presidential election by harming Hillary Clinton's chances of success and supporting Donald Trump at the direction of the Kremlin," it said.

"IRA social media activity was overtly and almost invariably supportive of then-candidate Trump," it said.

The committee, led by Republican Senator Richard Burr, issued its report as Trump continues to insist that the story of Russian meddling is "fake news" and a "Russian witch hunt hoax" peddled by Democrats.