WASHINGTON — The White House declared Tuesday it will not cooperate with what it termed the “illegitimate” impeachment probe by House Democrats, sharpening the 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.

“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 currently objecting that the House did not formally vote to begin the impeachment inquiry into Trump. It also claims that Trump’s due process rights are being violated and is 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 formal impeachment inquiry vote.

Schiff, commenting before the White House letter was released, said, “For this impeachment inquiry we are determined to find answers.”

The Constitution states the House has the sole power of impeachment, and that the Senate has the sole power to conduct impeachment trials. It specifies that a president can be removed from office for “treason, bribery, or other high crimes and misdemeanors,” if supported by a two-thirds Senate vote. But it offers little guidance beyond that on proceedings.

The White House letter marks the beginning of a new 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.

Earlier Tuesday, 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.

The House followed up Tuesday afternoon with subpoenas for Sondland’s testimony and records.

As lawmakers seek to amass ammunition to be used in an impeachment trial, the White House increasingly believes all-out warfare is its best course of action.

“What they did to this country is unthinkable. It’s lucky that I’m the president. A lot of people said very few people could handle it. I sort of thrive on it,” Trump said Monday at the White House. “You can’t impeach a president for doing a great job. This is a scam.”

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.

Pelosi said thwarting the witness testimony on Tuesday was an “abuse of power” in itself by the president.

A senior administration official told reporters that no additional witnesses under its purview will be permitted to appear in front of Congress or comply with document requests, saying the policy under the current circumstances is that the administration will have “a full halt” because “this is not a valid procedure” for an impeachment inquiry. The official spoke on the condition of anonymity to discuss the administration’s position.

The letter mounts a sweeping and aggressive attack on the House proceedings and signals a battle ahead over whether the president is receiving the legal protections he and his lawyers believe he deserves.

The White House is claiming that Trump’s constitutional rights to cross-examine witnesses and review all evidence in impeachment proceedings extend even to House investigations, not just a potential Senate trial. It also is calling on Democrats to grant Republicans in the House subpoena power to seek evidence in the president’s defense.

The White House letter came as a federal judge heard arguments Tuesday in a separate case on whether the House had undertaken a formal impeachment inquiry despite not having taken an official vote and whether the inquiry can be characterized, under the law, as a “judicial proceeding.”

That distinction matters because while grand jury testimony is ordinarily secret, one exception authorizes a judge to disclose it in connection with a judicial proceeding. House Democrats are seeking grand jury testimony from special counsel Robert Mueller’s Russia investigation as they conduct their impeachment inquiry.

“The House under the Constitution sets its own rules, and the House has sole power over impeachment,” Douglas Letter, a lawyer for the House Judiciary Committee, told the court.