WASHINGTON (AP) - The White House declared Tuesday it will not cooperate with what it termed an "illegitimate" impeachment probe by House Democrats, setting up a constitutional clash between President Donald Trump and Congress.

Trump attorneys sent a letter to House leaders bluntly stating their refusal to participate in the quickly moving impeachment investigation. The letter threatens to cease cooperation with Capitol Hill on key oversight matters, accusing lawmakers of formulating their probe "in a manner that violates fundamental fairness and constitutionally mandated due process."

"Given that your inquiry lacks any legitimate constitutional foundation, any pretense of fairness, or even the most elementary due process protections, the Executive Branch cannot be expected to participate in it," White House Counsel Pat Cipollone wrote.

The White House is objecting that the House did not formally vote to begin the impeachment inquiry into Trump and is also attacking the conduct of House intelligence committee Chairman Adam Schiff.

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 vote.

The letter comes the same day that Trump intensified 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.

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

The White House has embarked on a newly clear strategy to counter the impeachment threat to Trump: Stall. Obfuscate. Attack. Repeat.

Trump aides have been honing their approach after two weeks of what allies have described as a listless and unfocused response to the impeachment probe.

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