President Donald Trump called the imminent testimony by Robert Mueller a “Rigged Witch Hunt” after the Democrat-controlled House Judiciary Committee decided to let the former special counsel’s top aide assist him.

Mueller reportedly requested his former chief of staff Aaron Zebley be at the witness table while he is testifying before the House Judiciary Committee and the House Intelligence Committee on July 24.

In a tweet late Tuesday, July 23, the president said the unparalleled decision is a disgrace to the U.S. system, “unfair,” and “should not be allowed.”

“Just got back only to hear of a last minute change allowing a Never Trumper attorney help Robert Mueller with his testimony before Congress tomorrow. What a disgrace to our system. Never heard of this before. VERY UNFAIR, SHOULD NOT BE ALLOWED. A rigged Witch Hunt!” he tweeted.

Several hours later, President Trump posted another tweet, saying Mueller’s lawyer “represented the ‘basement server guy’ who got off free in the Crooked Hillary case” and repeating “This should NOT be allowed.”

Zebley is known as Mueller’s top aide, working closely with the former special counsel on the Russia probe for nearly two years.

Before the testimony, the Department of Justice (DOJ) sent its guidance to Mueller on July 22, saying the decision to testify was his to make, though the department agreed with his stated position that the testimony should be unnecessary under the circumstances. DOJ also reminded that any testimony must remain within the boundaries of his public redacted report.

Mueller is now no longer a Justice Department employee. He decided to formally close the special counsel’s office and resigned from the department after he made the first public statement about his Russia probe report in late May.

In the report, the Mueller team concluded that there was no evidence that the Trump campaign colluded or conspired with Russia. The former special counsel said he could not conclude President Trump was innocent of some crimes, but could not name any crime the president committed.

Such a result raised criticisms that Congress is wasting time and taxpayers money on the probe that President Trump frequently calls “Witch Hunt.”