With an assist from Grass.
Eugene Volokh tracked this down seven years ago. It remains a favorite story of mine.
I wanted to get the source for the "dark night of fascism is always descending in the United States and yet lands only in Europe," so I tracked it down to Tom Wolfe's "The Intelligent Coed's Guide to America," republished in Mauve Gloves & Madmen, Clutter & Vine (1976). In the process, I found a more extended discussion that struck me as worth repeating. Here's the relevant excerpt, from pp. 115-17 of the hardcover edition; it reports on a panel discussion at Princeton in 1965, in which the participants included Paul Krassner, editor of The Realist magazine, Günter Grass, and Wolfe:
The next thing I knew, the discussion was onto the subject of fascism in America. Everybody was talking about police repression and the anxiety and paranoia as good folsk waited for the knock on the door and the descent of the knout on the nape of the neck. I couldn't make any sense out of it. . . . This was the mid-1960's. . . . [T]he folks were running wilder and freer than any people in history. For that matter, Krassner himself, in one of the strokes of exuberance for which he was well known, was soon to publish a slight hoax: an account of how Lyndon Johnson was so overjoyed about becoming President that he had buggered a wound in the neck of John F. Kennedy on Air Force One as Kennedy's body was being flown back from Dallas. Krassner presented this as a suppressed chapter from William Manchester's book Death of a President. Johnson, of course, was still President when it came out. Yet the merciless gestapo dragnet missed Krassner, who cleverly hid out onstage at Princeton on Saturday nights. . . .
Support [for Wolfe's view that fascism wasn't coming to America] came from a quarter I hadn't counted on. It was Grass, speaking in English.
"For the past hour, I have my eyes fixed on the doors here," he said. "You talk about fascism and police repression. In Germany when I was a student, they come through those doors long ago. Here they must be very slow."
Grass was enjoying himself for the first time all evening. He was not simply saying, "You really don't have so much to worry about." He was indulging his sense of the absurd. He was saying: "You American intellectuals — you want so desperately to feel besieged and persecuted!"
He sounded like Jean-François Revel, a French socialist writer who talks about one of the great unexplained phenomena of modern astronomy: namely, that the dark night of fascism is always descending in the United States and yet lands only in Europe.
Not very nice, Günter! Not very nice, Jean-François! A bit supercilious, wouldn't you say! . . .