The Trump White House says it will not participate in Congress's ongoing impeachment inquiry, stepping up a political and legal standoff between the two branches of government.

In a blistering 8-page letter to Democratic congressional leaders, including House Speaker Nancy Pelosi, White House Counsel Pat Cipollone repeatedly mocks the Democrats' process, saying it "lacks any legitimate constitutional foundation" and is simply an effort to "nullify the outcome of the democratic process" by reversing the outcome of the 2016 election and influence the upcoming 2020 election.

Cipollone argues that, because for the first time in history, there has been no formal vote to begin an impeachment inquiry, they are not required to comply; Pelosi has said that an official vote is not necessary under House rules to proceed, letting instead the relevant House committees begin and continue investigations.

"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," Cipollone writes. "Because participating in this inquiry under the current unconstitutional posture would inflict lasting institutional harm on the Executive Branch and lasting damage to the separation of powers, you have left the President no choice."

On a conference call with reporters, however, a senior administration official would not commit to cooperating with Democrats even if there is a formal vote.

The letter comes after the Trump administration this morning laid down a marker against the Democrats' impeachment efforts by blocking Gordon Sondland, Trump's ambassador to the European Union, from testifying before Congress. Trump tweeted that he wasn't going to subject Sondland to questions from a "kangaroo court." Congressional Democrats want answers about Sondland and others' participation in the growing scandal over Ukraine and whether Trump sought to pressure a foreign government to investigate his potential 2020 rival, former Vice President Joe Biden.

In a statement, White House Press Secretary Stephanie Grisham said that the efforts by Democrats are "purely political" and "ignores the fundamental rights guaranteed to every American. "

"These partisan proceedings are an affront to the Constitution—as they are being held behind closed doors and deny the President the right to call witnesses, to cross-examine witnesses, to have access to evidence, and many other basic rights," Grisham said.

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