A defiant White House declared war on Tuesday against the impeachment investigation of Donald Trump, saying neither the president nor his administration will co-operate.
It blasted the process as partisan, illegitimate and unconstitutional.
In a fiery letter, the White House challenged House Speaker Nancy Pelosi and the chairmen of three congressional panels to proceed with their inquiry to oust Mr Trump without co-operation.
"Put simply, you seek to overturn the results of the 2016 election and deprive the American people of the president they have freely chosen," 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.
"President Trump cannot permit his administration to participate in this partisan inquiry under these circumstances."
The inquiry is looking into whether Mr Trump abused his office by seeking a probe by Ukraine into his presidential rival, Joe Biden.
Washington is now on a path towards a constitutional crisis, with the possibility of a court showdown as the nation gears up for the 2020 election.
The White House said its main objection was the fact the House of Representatives had not held a formal vote to launch the inquiry.
Democrats say it is not needed because the impeachment process is in its earliest stages, similar to to gathering evidence for an indictment.
Only afterward would the Democrats call a vote. If most House members backed impeachment, the matter shifted to a trial in the Senate, which is controlled by Republicans.
Earlier on Tuesday, Mr Trump barred a key witness, ambassador to the EU Gordon Sondland, from speaking to Congress, calling it a "totally compromised kangaroo court".
In response, the Democrats issued Mr Sondland with a subpoena that "compels" him to appear on October 16.
Ms Pelosi formally launched the impeachment inquiry last month after revelations that Mr Trump pressured Ukraine's President Volodymyr Zelenskiy in a phone call On July 25.
The top Democrat in Congress said that preventing Mr Sondland from testifying showed that "the president is obstructing Congress from getting the facts that we need".
Mr Biden, the former vice president who is seeking the 2020 Democratic nomination, joined the chorus of condemnation, tweeting that Mr Trump "must stop stonewalling Congress".
Mr Sondland, a major donor to Mr Trump's 2016 presidential campaign, was one of a handful of US diplomats on a text message chain between July and September that go to the heart of the investigation.
The messages involving Mr Trump's personal lawyer Rudy Giuliani show them co-ordinating to press Kiev into investigating Mr Biden, as Ukraine sought US military aid and access.
House intelligence committee chairman Adam Schiff said investigators had learnt that Mr Sondland had text messages or emails on a personal device that were "deeply relevant" to the probe, but that the state was withholding them.
"The failure to produce this witness, the failure to produce these documents" was "additional strong evidence of obstruction of the constitutional functions of Congress", Mr Schiff said.
Politicians have a chance on Friday to hear from another key witness in the Ukraine scandal, former US ambassador to Kiev Marie Yovanovitch, who is scheduled to appear before the committee.
US media reported that Mr Trump removed her from her post because she opposed his efforts to have Ukraine investigate Mr Biden.
It was unclear whether Mr Trump will block Ms Yovanovitch's testimony.
After the White House's letter, members of the Trump administration will not be authorised to testify in Congress and will ignore subpoenas, a senior official said.
He said the White House was "definitely avoiding saying there's no way we'd ever co-operate", but he declined to discuss "hypothetical situations" that might bring a change.
Republicans were largely silent after the White House's gambit, but Democrats were quick to portray a reckless president defying legislators.
"The president's letter shows he believes he's above the law," 2020 White House hopeful Beto O'Rourke tweeted. "It's on Congress to prove him wrong."
