Former Attorney General Eric Holder called a possible dismissal of special counsel Robert Mueller by President Trump an “absolute red line” that cannot be crossed.

“ABSOLUTE RED LINE: the firing of Bob Mueller or crippling the special counsel’s office. If removed or meaningfully tampered with, there must be mass, popular, peaceful support of both. The American people must be seen and heard - they will ultimately be determinative,” Holder tweeted Sunday.

The tweet comes a day after the Trump campaign sought to discredit Mueller’s investigation by sending a letter to Congress alleging members of the transition team’s Fourth Amendment rights against illegal search and seizure had been violated.

The special counsel’s office obtained tens of thousands of emails from the Trump transition team through the General Services Administration. The Trump campaign argued the government is not the rightful owner of the emails, instead the holders of the accounts are the owners.

The special counsel’s office argued it went through the proper channels to obtain the emails, likely asking the GSA to turn over the documents.

Mueller’s investigation has been put under the microscope by Republicans in the last few days.

The Department of Justice called in reporters to view text messages between two FBI agents showing anti-Trump bias, one of whom was fired from the Mueller investigation in July after the text messages were discovered. Republicans have argued this is proof the investigation is partisan and led by Democrats.

Mueller is a registered Republican who previously served as FBI director after being appointed by President George W. Bush.