Senate Democratic leader Chuck Schumer and other Democrats have called for Whitaker to recuse himself from overseeing the Mueller investigation.
He tweeted on Thursday that investigators were "threatening" people to provide "the answers they want".
"They are a disgrace to our Nation and don't care how many lives (they) ruin". "They won't even look at all of the bad acts and crimes on the other side", he said.
Mr Trump may simply be reflecting the growing sense that Mr Mueller is poised for new action after a pre-election quiet period. It's worth noting that multiple reports suggest that the president has been unusually angry and dour in the past week. After a brief mid-term interlude that included dire warnings about refugee caravans in Mexico and mix-and-match endorsements of Republican candidates, the looming Russian Federation investigation is back on the president's mind - and his Twitter feed. "Nothing I've seen would require him to be recused".
Following a tumultuous week since the midterm elections, President Trump on Thursday angrily unloaded on a number of favorite targets - while calling the U.S. "the envy of the world" because of his presidency. Add Russia Investigation as an interest to stay up to date on the latest Russia Investigation news, video, and analysis from ABC News.
The president's son, Donald Trump Jr, met a Russian lawyer during the campaign who was said to have "dirt" on Mrs Clinton, and adviser George Papadopoulos has admitted lying to the Federal Bureau of Investigation about meetings with alleged go-betweens for Russia. Such an indictment would hit much closer to home than any previous moves by the Mueller team, and the fallout would be unpredictable.
President Donald Trump with Chief Paul Cell, of the International Association of Chiefs of Police, left, during an announcement of his support for a criminal justice reform bill, called the First Step Act, at the White House in Washington, on November 14.
Trump has recently taken to pushing the bogus talking point that Mueller was not confirmed by the Senate, and legal experts told INSIDER the claim is "flat out wrong" because Mueller is an "inferior officer" - not a "principal officer" who reports directly to the President - and therefore does not need to be confirmed by the Senate.