Goldberg is certainly right when he says that most academics have willfully ignored modern liberalism's progressive-fascist roots, although scholars such as James Ceaser, John Marini, and others (including me) have in fact been calling attention to the progressive origins of modern liberalism for the past 20 years. Liberal Fascism clearly draws from these works but makes surprisingly little reference to them, even in a few instances when the book's observations sound awfully familiar.
Jonah would never plagiarize someone else. Sometimes it might seem that way, such as when he reviewed the movie "Cloverleaf," saying "It’s “Godzilla” for the MySpace generation and nothing more" after a Boston Globe critic said '"Cloverfield" captures the chronic self-absorption of the Facebook generation with breathless, cleverly recycled media savvy, and then it stomps that self-absorption to death.' But that doesn't mean Jonah copied anything, just that he's not very original.
So what if some critics who have been discussing "liberal fascism" did the work first? Did they put a Hitler smiley face on the cover? I think not! Jonah promises to review the review soon, maybe after lunch or dinner, so I'm sure he'll just say he doesn't understand why anyone would make such a claim, after he's written his book with such care and attention to detail. There's just no way he'd not attribute something properly, and if he did it must have been the research assistant who did it.