Senator Sheldon Whitehouse saw this coming way back in August. Not the specific attacks—President Donald Trump randomly strafing the Justice Department and the judicial system in a series of tweets and rants. But the Rhode Island Democrat, himself a former federal prosecutor, knew that as the Russia investigations crept closer to the White House Trump would very likely try to undermine the most credible and dangerous probe into foreign efforts to throw the 2016 presidential election. So this past summer, Whitehouse co-sponsored a bill to shield special counsel Robert Mueller from being fired.

The bill has stalled, thanks to the Republican majority. But Whitehouse and his compatriots in both houses of Congress are feeling an increased urgency to prepare for the next big shoe to drop. Conversations in Capitol Hill hallways and meeting rooms increasingly turn to what to do in the event that Trump tries to whack Attorney General Jeff Sessions or Mueller or both. Democrats are quietly lobbying Republicans to be ready to move quickly if Trump pulls the trigger. “Firing Mueller, that’s a showstopper in terms of the last semblance of normalcy being stripped away. That would really be brute authoritarianism at work. And I think a great number of my Republican colleagues would find that to be a bridge too far,” Whitehouse says. “If it looks like we’re moving into a Saturday Night Massacre redux, then it’s going to be incumbent upon the Republican leadership in both houses to act expeditiously.”

Senator Martin Heinrich, a member of the Senate Intelligence Committee, points to another alarming indicator, one being fomented by the legacy of Nixon aide Roger Ailes. “On Fox News, you see what looks to be a concerted effort to discredit someone who not very long ago was getting glowing praise across the board, from the Newt Gingriches of the world, from all of these Republicans who had worked with Mueller when he was F.B.I. director,” the New Mexico Democrat says. “That is an early warning sign that we ought to be taking this seriously.”

The original Saturday Night Massacre, in 1973, came when President Richard Nixon tried to short-circuit the Watergate investigation. He ordered Attorney General Elliot Richardson to fire special prosecutor Archibald Cox; Richardson refused, and quit, as did Deputy Attorney General William Ruckelshaus. Next in the chain of command was Solicitor General Robert Bork, who was hustled to the White House in a limousine and who went along with Nixon’s demands. No one knows what the current officeholders would do if confronted with the same choices: Sessions is officially recused from involvement with the Russian investigation, leaving the tough decisions to his deputy, Rod Rosenstein, and possibly Associate A.G. Rachel Brand and Solicitor General Noel Francisco. But in contrast to 1973, the ripples would likely spread farther and faster in the Justice Department. “You would see a mix of resignations and open defiance by career officials at D.O.J.,” says Matthew Miller, a department spokesman during the Obama administration. “From people I talk to, they are upset by two things. One, that the president is attacking the independence of the department. But what I hear, over and over again, is how angry they are that the leadership of the department doesn’t do a thing about it. Trump is trying to breach what is the most sacred principle of the Justice Department, the wall between politics and law enforcement, and the attorney general and the deputy attorney general have said nothing to tell him to stop. Ultimately, though, if Trump moves against Mueller, this is when you’d have to ask the political system to do its job. That would be an impeachable offense, and it would really be time for Congress to step up and say, ‘Enough, we’re not going to let you turn this country into a banana republic.’”