The White House notified the US House on Tuesday that the Trump administration would not participate in what it is calling the Democrats' "illegitimate" impeachment probe, paving the way for a constitutional clash with Congress. 

In a letter sent to House leaders, Trump administration attorneys clearly stated their refusal to participate in the inquiry.

The letter accused Democratic 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 specifically objects that the House did not formally vote to begin the impeachment inquiry into Trump, and said because of this it "lacks the necessary authorization for a valid impeachment proceeding."

Cipollone also said the Trump administration would not cooperate because the inquiry has not met "due process rights" of the president. 

In response, House Speaker Nancy Pelosi insisted the House had constitutional rights to conduct oversight of the executive branch regardless of a formal impeachment inquiry vote. 

"This letter is manifestly wrong, and is simply another unlawful attempt to hide the facts of the Trump Administration's brazen efforts to pressure foreign powers to intervene in the 2020 elections," Pelosi said in a statement.    

"Despite the White House's stonewalling, we see a growing body of evidence that shows that President Trump abused his office and violated his oath to 'protect, preserve and defend the Constitution,'" she added.

House subpoenas US ambassador to EU

The letter from the White House Council came shortly before the House subpoenaed the US ambassador to the EU, Gordon Sondland.

In a letter to Sondland, the Democratic chairs of three House committees said the subpoena compels him to appear on October 16 to answer questions on his role in Trump's interactions with the Ukraine government.

Sondland agreed to freely testify before members of three House committees earlier in the day, but the State Department blocked the ambassador's appearance, according to Sondland's lawyer, Robert Luskin.

Luskin said his client was "profoundly disappointed" that he would not be able to testify.

"He stands ready to testify on short notice, whenever he is permitted to appear," Luskin said in a statement.

The chairman of the House Intelligence Committee, Adam Schiff, said that blocking Sondland's testimony was "additional strong evidence" that Trump and Secretary of State Mike Pompeo were obstructing the probe.

Leaked text messages

Sondland drew the attention of the lawmakers after a whistleblower leaked the envoy's text messages on interactions between Washington and Kyiv. The president faces accusations that he withheld security aid to Ukraine to pressure his Ukrainian counterpart to probe Hunter Biden, the son of former Vice President Joe Biden, for his role as a board member at Ukraine's largest gas company. Biden is one of the Democrats' top candidates to face Trump in the 2020 presidential election.

According to Trump critics, Sondland's messages show that the ambassador worked with another Trump envoy to pressure Ukraine into complying with the president's wishes. The American officials allegedly conditioned a meeting in Washington between Trump and Ukraine's new president, Volodymyr Zelenskiy, on his cooperation in probing the Bidens and examing another debunked theory about election interference in the 2016 election. 

The texts also included a message from Kurt Volker, formerly Trump's special envoy to Ukraine, to a Zelenskiy advisor.

"Heard from the White House," Volker wrote in July this year.

"Assuming President Z [Zelenskiy] convinces trump [sic] he will investigate/'get to the bottom of what happened' in 2016, we will nail down a date for visit to Washington."

It also was not immediately clear why Sondland, a former hotel magnate who serves as the US envoy to the EU, was involved in dealings with Ukraine, which is not an EU member.

'Kangaroo court'

The texts also include a message sent to Sondland, where another diplomat wrote: "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland replied that Trump had been "crystal clear no quid pro quos of any kind."

"I suggest we stop the back and forth by text," the ambassador added.

Trump has denied any wrongdoing, but also publicly urged China to investigate the Bidens.

The State Department did not immediately explain why it blocked Sondland from appearing before the House Foreign Affairs, Intelligence and Oversight committees. However, Trump himself said he did not want to have the envoy testify "before a totally compromised kangaroo court."

The House members are scheduled to interview a former ambassador to Ukraine, Marie Yovanovitch, behind closed doors on Friday. Trump recalled Yovanovitch in May, months before her term was set to expire, after several Trump allies publicly questioned her loyalty to the president.

cw,kmm/dj/se (AP, Reuters)

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