The former vice president, who had already ruled out voluntarily testifying at the impeachment proceedings, told NPR in an interview published on Monday he would not cooperate with a subpoena.

"No, I'm not going to let you take the eye off the ball here. Everybody knows what this is about," Biden said. "This is a Trump gambit he plays. Whenever he's in trouble he tries to find someone else to divert attention to."

When pressed, Biden stood resolute, claiming there was not "one scintilla of evidence" he did anything wrong.