US President Donald Trump on Monday branded Democrats a "disgrace" for holding impeachment proceedings while he attends a NATO summit in Britain and rejected participating in what he called "a hoax."
House Democrats are due to move forward this week with their case to make the real estate mogul only the third president ever to be impeached.
The House Judiciary Committee will hold a hearing on Wednesday at which experts will weigh whether Trump's alleged wrongdoing in pressuring Ukraine to investigate domestic political foe Joe Biden meet the constitutional bar for impeachment.
"The Democrats, the radical-left Democrats, the do-nothing Democrats, decided when I'm going to NATO -- this was set up a year ago -- that when I'm going to NATO, that was the exact time," Trump said angrily on departing the White House.
"It's an absolute disgrace what they're doing to our country," he said before his plane touched down in London.
"The whole thing is a hoax. Everybody knows it."
Trump's chief White House lawyer, Pat Cipollone, told the Democratic leader of the Judiciary Committee, Jerry Nadler, on Sunday that he was rejecting an invitation to send representatives to the session.
"We cannot fairly be expected to participate... while it remains unclear whether the Judiciary Committee will afford the President a fair process through additional hearings," Cipollone wrote.
Cipollone did not rule out White House participation in subsequent hearings. Throughout the drama, however, Trump has opted for stone-walling and flat-out resistance to what his supporters say amounts to a "coup."
Nadler told the White House on Monday that if "there is nothing to hide," then Trump should cooperate and "provide any exculpatory information that refutes the overwhelming evidence of his abuse of power."
Trump got a boost Monday from an interview in which Ukrainian President Volodymyr Zelensky insisted he had not come under pressure from Trump.
The US president is accused of brazenly holding up hundreds of millions of dollars of military aid to Ukraine to force Zelensky to announce a politically embarrassing probe into Biden, one of the lead Democrats challenging Trump for the presidency in 2020.
A stream of high-level diplomats and several White House officials have testified in Congress about Trump's back-channel communications with Zelensky.
But Zelensky said Monday that he'd not been forced into anything.
"I did not speak with US President Trump in those terms: 'you give me this, I give you that,'" Zelensky said in an interview with several publications, including Germany's Der Spiegel magazine.
Trump says he was right to raise his concerns over alleged corruption by Biden and his son Hunter, who was controversially named to the board of a Ukrainian energy company accused of corrupt practices.
And he repeatedly echoed the Zelensky interview on Monday, saying "the Ukrainian president came out and said very strongly that President Trump did absolutely nothing wrong."
"That should be case over," Trump said.
Democratic lawmakers say that Zelensky, a neophyte politician facing armed conflict with Russia, was desperate to please Trump from the outset and remains unable to speak his mind for fear of losing support.
The House Intelligence Committee and its Democratic leader Adam Schiff has led the impeachment charge from the start.
The committee is expected to approve its investigation report on Tuesday in a closed-door vote set for 6:00 pm (2300 GMT).
Trump's Republican backers on the intelligence committee on Monday released a 110-page rebuttal of the Democrats's case.
"The evidence does not establish any impeachable offense," it said.
Shortly after landing in Britain, Trump tweeted: "Great job! Radical Left has NO CASE."