Pelosi had formally launched the impeachment inquiry last month after revelations Trump pressured Ukraine's President Volodymyr Zelensky in a July 25 phone call.
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 for a court showdown just as the nation gears up for the 2020 election.
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.
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 chairman Adam Schiff said investigators have learned that Sondland has text messages or emails on a personal device that are "deeply relevant" to the probe, but that State is withholding them.