The president went on to name several key players in the Ukraine scandal at the heart of House Democrats’ impeachment investigation who have refused to provide testimony regarding Trump’s decision to withhold security assistance from the Eastern European nation and push for foreign probes into his domestic rivals.

Former national security adviser John Bolton “is a patriot and may know that I held back the money from Ukraine because it is considered a corrupt country, & I wanted to know why nearby European countries weren’t putting up money also,” Trump tweeted.

“Likewise, I would … love to have Mike Pompeo, Rick Perry, Mick Mulvaney and many others testify about the phony Impeachment Hoax,” the president wrote, invoking his secretary of State, outgoing Energy secretary and acting White House chief of staff, respectively.

“It is a Democrat Scam that is going nowhere but, future Presidents should in no way be compromised,” Trump concluded. “What has happened to me should never happen to another President!”

Asked about Trump's tweets and whether he was considering testifying, Pompeo told reporters during a news conference at the State Department: “When the time is right, all good things happen.”

The House Intelligence Committee last week finished a series of open hearings that saw damaging public testimony from a dozen witnesses who described a shadow foreign policy in Ukraine spearheaded by Rudy Giuliani, Trump’s personal attorney, and administration officials pursuing the president’s political interests.

Republicans in Congress have repeatedly criticized the impeachment proceedings for a lack of first-hand testimony documenting direct conversations with Trump, while Democratic lawmakers have pointed to the White House’s efforts to stymie government employees’ compliance with the inquiry.