He said firing Mueller would trigger the type of constitutional crisis not seen in more than four decades.

At the height of the Watergate scandal, Nixon’s firing of Cox sparked an intense backlash known as the Saturday Night Massacre. The outcry was so great that Nixon was forced to name a new special prosecutor, Leon Jaworski. Nixon resigned the presidency less than a year after dismissing Cox.

Sen. Rob Portman, R-Ohio, said he supports the Mueller investigation and thinks it’s important that it reach a conclusion. “We know that Russia has tried to influence our elections, since long before 2016, and they’re going to continue to try to do so moving forward unless we do something about it,” he said.

Sen. Sherrod Brown, D-Ohio, who has made clear he would oppose any attempt to fire Mueller, said “the investigation needs to be done right and it needs to be done swiftly.”