At the end for Richard Nixon, after all the mounting evidence in the Watergate scandal, after both special prosecutors, after all the White House indictments, after the guilty pleas, after the obstruction efforts fell apart, after all the court fights, after all the damaging revelations in outlets like the Washington Post, Time and the Los Angeles Times, after all the impeachment hearings, it all came down to Barry Goldwater.

It’s easy, nearly 50 years after Watergate, to forget that Nixon’s ignominious departure from the White House was hardly a foregone conclusion. The Republican Party had stuck closely with Nixon even through the darkest days of the Watergate scandal; even as its lawmakers whispered behind closed doors about his guilt and even as public opinion polls showed Nixon dragging down their party, they had toughed it out—past the indictments of his top aides, past the courts batting back one attempt at obstruction after another, even after Nixon’s attacks on and ultimate firing of the special prosecutor targeting him.

It wasn’t until August 6, 1974, at the regular Senate Republican Conference lunch that Barry Goldwater fumed to his colleagues: “There are only so many lies you can take, and now there has been one too many. Nixon should get his ass out of the White House—today!”

Hours later, he ventured to the White House to tell Nixon to resign.

And, amazingly, Nixon did. For Nixon knew that when Goldwater threw in the towel, it really was over.

Examining this critical turning point in Nixon’s presidency and the arc of the larger Watergate scandal carries with it today important lessons about the impeachment trial President Donald Trump is now undergoing. It also raises the all-important question of whether there’s a Barry Goldwater moment ahead in Trump’s future. Is there even a figure in the GOP left today to carry such a message to a White House under siege? Is there even a figure in the GOP whom Trump respects enough to listen to?