Washington – A defiant White House declared war on Tuesday against the impeachment investigation of US President 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 chairpersons 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 standoff set Washington on a path toward a constitutional crisis and into unknown territory, with the potential for a court showdown just as the nation gears up for the 2020 election.The White House's chief 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, equivalent to gathering evidence for an indictment.Only afterward would the Democrats call a vote. If a majority of House members back impeachment, the matter shifts to a trial in the Senate, currently controlled by Republicans.Top Senate Republican Mitch McConnell expressed support for Trump's strategy, tweeting that the House had failed "to follow the same basic procedures that it has followed for every other President in our history".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.'Stonewalling Congress'Pelosi formally launched the impeachment inquiry last month after revelations Trump pressured Ukraine's President Volodymyr Zelensky in a July 25 phone call.Biden, the former vice president who is seeking the 2020 Democratic nomination, joined the chorus of condemnation, tweeting that Trump "must stop stonewalling Congress".Sondland, a major donor to Trump's 2016 presidential campaign, was one of a handful of US diplomats on a text message chain between July and September that goes to the heart of the investigation.The messages involving Trump's personal lawyer Rudy Giuliani show them coordinating to pressure Kiev into investigating Biden, as Ukraine sought US military aid and access.House Intelligence Committee chairperson Adam Schiff said investigators have learnt that Sondland has text messages or emails on a personal device that are "deeply relevant" to the probe, but that State is withholding them.'Evidence of obstruction'"The failure to produce this witness, the failure to produce these documents" was "additional strong evidence of obstruction," Schiff told reporters.Lawmakers have an opportunity 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 House Intelligence Committee.US media has reported that Trump removed her from her post because she opposed his efforts to get Ukraine to investigate Biden.It was unclear whether Trump will block Yovanovitch's testimony.Following the White House's letter, members of the Trump administration will not be authorised to testify in Congress and will ignore subpoenas, a senior administration official told reporters.He insisted that the White House was "definitely avoiding saying there's no way we'd ever cooperate," but he declined to discuss "hypothetical situations" in which a change might come.