The White House wrote a defiant letter to prominent Democrats Tuesday saying it will not cooperate with their “illegitimate and unconstitutional” impeachment inquiry into President Donald Trump’s dealings with Ukraine.

The eight-page letter, addressed to House of Representatives Speaker Nancy Pelosi and other senior Democrats, said the inquiry was unfair, had no basis in the U.S. Constitution and flouted due process and the rule of law.

“The president has done nothing wrong, and the Democrats know it,” Stephanie Grisham, press secretary at the Republican-held White House , said in a statement.

“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.”

The letter, written by Trump’s lawyer Pat Cipollone, accuses Democrats of seeking to “overturn the outcome” of the 2016 election while also trying to damage Trump’s chances of being re-elected in 2020.

“For these reasons, the executive branch cannot be expected to, and will not participate in, this exercise of partisan political theater,” added Grisham.

House lawmakers launched an impeachment inquiry against Trump on Sept. 24 following claims by a whistle-blower that the president had sought to pressure Ukraine to interfere in the 2020 U.S. presidential elections.

In a July 25 phone call, Trump allegedly made military aid contingent on Ukrainian President Volodymyr Zelensky launching a probe into Joe Biden, a former U.S. vice president, and his son, Hunter, a businessman, over unsubstantiated corruption allegations.

The elder Biden is a leading candidate in the race to win the Democratic nomination and challenge Trump in 2020. Trump, a Republican, has repeatedly denied any wrongdoing and accused Democrats of time-wasting.