President Trump said Tuesday he will not cooperate with the congressional impeachment inquiry, pushing the United States toward a constitutional crisis. The White House announced its decision in an eight-page letter addressed to House Democratic leaders, blasting the inquiry as partisan and unconstitutional.

The letter arrived just hours after the Trump administration ordered a key witness in the impeachment probe, Ambassador Gordon Sondland, not to appear for scheduled testimony on Capitol Hill. President Trump tweeted, “unfortunately he would be testifying before a totally compromised kangaroo court.” Text messages turned over to Congress reveal Sondland helped to mediate conversations between Ukrainian officials and Trump’s personal attorney, Rudy Giuliani, aimed at winning a promise by Ukraine’s president to investigate Joe Biden and his son in exchange for hundreds of millions of dollars of U.S. military aid that Trump had placed on hold.

On Capitol Hill, House Intelligence Committee Chair Adam Schiff said Tuesday Democrats would issue a subpoena ordering Sondland to appear before Congress and to hand over electronic messages related to the investigation. Schiff said the White House’s decision to stonewall the impeachment probe was further evidence of obstruction.

Rep. Adam Schiff: “This is one of the few impeachment inquiries in the history of our country. It goes to the core of whether the president abused his office to seek political help in his re-election campaign and did so to the detriment of our nation’s security, did so by effectively coercing a country that has been invaded by Russia to investigate a rival, and condition the relationship between this country and that country on whether they were willing to play ball.”