He was one of four members of Mr Trump's campaign staff who have been charged by Mr Mueller's team to date.
On Wednesday, Jan. 3, The New York Times reported that Paul Manafort, Trump's former campaign chairman, sued Mueller and requested a federal court to restrict the special counsel's authority.
Manafort's attorneys claimed that the counts Manafort is facing fall outside of what should have been the acceptable scope for Mueller's probe and should essentially be dismissed.
In response, a Justice Department spokesperson called the lawsuit "frivolous" but apparently didn't offer much detail as to how the department would proceed.
Manafort's attorneys have echoed the President's criticism that Mueller's investigation into Russian meddling in the 2016 election is pursuing crimes that never happened.
He alleges in his lawsuit that the investigation into "decade-old business dealings" is "completely unmoored" from the mandate Mueller was given when he was appointed in May by Deputy Attorney General Rod Rosenstein.
The focus is on a part of the Rosenstein order that says that Mueller may investigate "any matters that arose or may arise directly from the investigation". He also wants the court to prevent Mueller from further investigating business dealings unrelated to the central focus of the Russian Federation probe.
Manafort resigned from President Donald Trump's campaign in August of 2016 amid scrutiny over his lobbying work for pro-Russia Ukraine figures. (Flynn is also Trump's former national security adviser.) Gates has pleaded not guilty, while Papadopoulos and Flynn have pleaded guilty. But Trump has told reporters in recent weeks that he has no plans to order the firing of Mueller, and his lawyers have said they are cooperating with the probe.