I'd disagree. There is a great deal of frustration with how corporations are being pandered to by the politicos on both sides; and I think Sanders could take a huge chunk of the Centrists vs. any of the GOP crowd right now. I've even seen a significant number of people I'd describe as Center-Right making noises in favour of Sanders. The only truly anti- notes that I've seen so far are from the "He hasn't got a chance, he'll split the Left vote and doom us all to a Republican theocracy!" crowd, the Far Right "OMG COMMUNISM WE'RE ALL DOOMED!!!" crowd, and the looney Libertarian crowd.
Given that the GOP candidates right now appear to be Ted Cruz, Mike Huckabee, Rand Paul, Rick Santorum, Chris Christie, Carly Fiorina, and Jeb Bush.
Huckabee is a non-starter, since he's controversial even among Far Right nutcases. No way he's going to pick up Centrists after the "I would have been transexual in high school to shower with the girls" nonsense. The man only opens his mouth to exchange feet. Not familiar with Chris Christie; but Santorum's pretty much a pair of clown shoes at the moment, and only likely to get worse. Rand Paul only motivates pseudo-libertarian whackjobs, and is way too far out to even consider giving the nomination to. Fiorina's a no-name after her disastrous tenure at HP. And about the last thing I think this country is going to elect is Yet Another Bush.
Cruz seems to be the only candidate who could possibly go toe to toe with either Clinton or Sanders.