Sen. Susan Collins said Friday that the special counsel Robert S. Mueller III must have been fired.
Matthew Ma Whitaker, who has criticized Mueller's investigation and now oversees it.
"For these reasons, I believe that we should bring the Senate floor legislation to Donald Trump's ability to fire the special counsel," Collins said in a statement.
Mr. Mueller has to be unimpeded to complete his work, "she said.
The future of the Mueller trial is in Trump has ousted Attorney General Jeff Sessions and replaced him with Whitaker, a political loyalist who has speculated about ways to stop the effort.
Sessions had previously recruited himself from overseeing the investigation, leaving the day-to-day oversight to Deputy Attorney General Rod J. Rosenstein.
But while Rosenstein remains in charge of that oversight, Collins noted, "Mr. Whitaker has the authority to intervene at any time. "
Whitaker has no intention of recuperation himself from overseeing the sample, people close to him said.
Collins's statement came after Sen. Jeff Flake (R-Ariz.) announced that he and Sen. Christopher A. Coons (D-Del.) Mueller from being fired.
The Senate Majority Leader Mitch McConnell (R-Ky.) Has repeatedly called it unnecessary.
"The President has repeatedly said he's not going to dismiss the Mueller investigation," McConnell said Friday in Kentucky, according to the Associated Press. "He's said again, it's going to be allowed to finish. That happens to be my view. "