This is one offer acting White House Chief of Staff Mick Mulvaney is probably going to refuse.

House Democrats leading the impeachment investigation sent a letter to Mulvaney on Tuesday requesting a voluntary deposition from him on Friday. Of course, that doesn't mean Mulvaney will actually listen to Democrats' wishes.

Impeachment investigators are asking Mulvaney to testify because, as they put it in the letter, he "may have been directly involved" in President Trump and Rudy Giuliani's alleged effort to "withhold a coveted White House meeting and nearly $400 million in security assistance" by pressuring Ukrainian President Volodymyr Zelensky to investigate Trump's political rivals. That's what a July 25 call between Trump and Zelensky, in which Trump mentioned the Biden family, seemed to indicate. And with Mulvaney previously serving as the head of the Office of Management and Budget, he would've been directly involved in that aid freeze.

The request comes after Mulvaney admitted to a quid pro quo between the U.S. and Ukraine, saying it's something that happens "all the time" in foreign policy. He later took the comment back. Several other Trump associates who Democrats asked to testify for the House have refused to show up, including some officials who were under subpoena. Kathryn Krawczyk