The White House has ruled out cooperating in the impeachment inquiry into President Donald Trump, describing the probe launched by the Democrats in the House of Representatives as "constitutionally invalid" and an attempt to "overturn the results of the 2016 election."

No cooperation

In an eight-page letter sent to House Speaker Nancy Pelosi, the top U.S. Democrat, and the Democratic chairs of the House intelligence, foreign affairs and oversight committees, the White House indicated that it would not cooperate until a formal vote to launch proceedings was held in the House of Representatives.

Pelosi argues the impeachment inquiry she launched is constitutional and that no House vote is yet needed because the impeachment process is in its earliest stages, equivalent to gathering evidence for an indictment.

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The letter, signed off by White House counsel Pat Cipollone, said Trump has been denied basic due process rights, such as to cross-examine witnesses, call witnesses to testify, receive transcripts of testimony, and access evidence.

"All of this violates the Constitution, the rule of law, and every past precedent," the letter said, adding that the Supreme Court has recognized due process protections apply to all congressional investigations.

Whistleblower worries

House Democrats are working to protect the identity of the person who has accused Trump of impropriety in his dealings with Ukraine, but the White House argued that the whistleblower should be cross-examined.

"There shouldn't be a situation where you can have a primary witness, an accuser in an impeachment inquiry, and the president never able to know who the accuser is and never able to cross examine him," a senior administration official told Reuters.

The letter argues – a contention denied by the Democrats – that the impeachment effort is simply aimed at reversing the result of the 2016 presidential election and influencing the 2020 election.

"Put simply, you seek to overturn the results of the 2016 election and deprive the American people of the president they have freely chosen," the letter reads.

Sondland subpoenaed

In another dramatic day in Washington, Trump also refused to allow Gordon Sondland, the U.S. ambassador to the EU, to testify before Congress, prompting a subpoena from House Democrats.

The president described the process as a "totally compromised kangaroo court." In response, Democrats served Sondland with a subpoena that "compels" him to appear on October 16.

Pelosi told reporters preventing Sondland from testifying shows that "the president is obstructing Congress from getting the facts that we need."

Sondland, a major donor to Trump's 2016 presidential campaign, is a key witness. 

He was one of a handful of U.S. diplomats on a text message chain showing apparent coordination to pressure Kiev into investigating Joe Biden, a leading contender for the Democratic 2020 presidential nomination, as the European country sought U.S. military aid and access.

What happens next?

Former U.S. ambassador to Kiev Marie Yovanovitch is scheduled to appear before the House Intelligence Committee on Friday. U.S. media has reported that Trump removed her from her post because she opposed his efforts to get Ukraine to investigate Biden. It is unclear whether her testimony will be blocked.

Following the White House's letter, members of the Trump administration will not be authorized to testify in Congress and will ignore subpoenas, a senior administration official, speaking on condition of anonymity, 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.

(With input from Reuters, AFP)