Mr. McGahn used that phrase when a staff lawyer for House Democrats grilled him at length about Mr. Trump’s efforts to get him to tell the deputy attorney general at the time, Rod J. Rosenstein, to remove Mr. Mueller over a dubious claim that the special counsel had a conflict of interest — which Mr. McGahn refused to do, believing it could “cause this to spiral out of control.” After Mr. Trump called him at home on a Saturday in 2017 to pressure him again to tell Mr. Rosenstein to oust Mr. Mueller, for example, Mr. McGahn testified, he was deeply concerned. “After I got off the phone with the president, how did I feel?” he said. “Oof. Frustrated, perturbed, trapped. Many emotions.” Fearing that conveying the directive might instead prompt Mr. Rosenstein to resign and touch off a crisis akin to President Nixon’s Saturday Night Massacre during the Watergate scandal, Mr. McGahn instead prepared to resign if Mr. Trump did not relent. He told several colleagues at the White House about his intention, although not Mr. Trump himself. But the crisis instead blew over for a time.