GOP Sen. Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (Maine) on Wednesday warned the Trump administration against interfering with special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's Russia investigation, saying he "must be allowed" to finish it.

Collins, a member of the Senate Intelligence Committee, also said that Mueller "must be allowed to complete his work without interference" regardless of who is the attorney general.

It is imperative that the Administration not impede the Mueller investigation. I’m concerned Rod Rosenstein will no longer be overseeing the probe. Special Counsel Mueller must be allowed to complete his work without interference—regardless of who is AG. — Sen. Susan Collins (@SenatorCollins) November 7, 2018

Deputy Attorney General Rod Rosenstein was put in charge of the investigation after Sessions recused himself last year.

"I want to thank Jeff Sessions for his service to our country as Attorney General. Under Acting Attorney General Matthew Whitaker, it is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded," he said in a tweet.

I want to thank Jeff Sessions for his service to our country as Attorney General. Under Acting Attorney General Matthew Whitaker, it is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded. — Mitt Romney (@MittRomney) November 7, 2018

Romney has gained attention both nationally and within the Senate GOP caucus as a someone who could push back against Trump once he comes to Washington next year.

Romney and Collins's pushback on Wednesday comes after Trump announced that he was ousting Sessions from the top Justice Department spot and that Whitaker, his chief of staff, will take over in an acting role including overseeing the Russia probe.