The chairmen of three House committees leading the impeachment investigation said they would compel testimony from US Ambassador to the European Union Gordon Sondland, a Trump donor who started his diplomatic job in July.
It lists examples, including that the president has been denied basic rights, such as the right to cross-examine witnesses and to have access to evidence. "In order to fulfill his duties to the American people, the Constitution, the Executive Branch, and all future occupants of the Office of the Presidency, President Trump and his Administration can not participate in your partisan and unconstitutional inquiry under these circumstances", Cipollone wrote in the letter addressed to House Speaker Nancy Pelosi (D-CA), House Intel Chair Adam Schiff (D-CA), House Foreign Affairs Chair Eliot Engel (D-NY) and House Oversight Chair Elijah Cummings (D-MD).
Cipollone also accused Congress of threatening executive branch officials with punishment for exercising "fundamental constitutional rights and prerogatives".
The impeachment inquiry is focused on a whistle-blower's allegations that Trump used almost $400m in U.S. military aid to secure a promise from Ukraine's president to investigate Biden, a leading candidate for the 2020 Democratic presidential nomination, and his son.
Later Tuesday, House Speaker Nancy Pelosi issued a statement in response to the White House letter, pointing out that Cipollone made "false claims about the Constitution, House rules and House precedent". Now, he is trying to make lawlessness a virtue.
"I presume the calculation is that congressional Republicans are more likely to stick with Trump over stonewalling Congress than after they hear whatever Sondland would say under oath", she tweeted.
Attorney Susan Hennessey, the Executive Editor of Lawfare, surmised that preventing Sondland from testifying was a strategic maneuver by the White House.
Progressive activists have been credited for keeping the heat on lawmakers to hold Trump accountable for his misconduct, and advocacy groups are continuing to mobilize to ensure House Democrats follow through with their inquiry.
House Speaker Nancy Pelosi on Tuesday said President Trump is "obstructing Congress from getting the facts" it needs in the impeachment inquiry after his administration blocked a United States diplomat from testifying before three committees. Sondland was involved as Trump urged the country to investigate political rival Joe Biden's family and the 2016 election. Now they are stating outright that the president "personally ordered the obstruction of a lawful Congressional inquiry".
"President Trump and his administration reject your baseless, unconstitutional efforts to overturn the democratic process", the eight-page letter read.