Yet she still was polling at 50-60% nationally, in polls that polled both Democrats and Republicans. Trump has been attacking Bernie at his rallies, and although they don't mention him by name all the time they certainly have been beating the drum about him being a Socialist. I believe I heard a ton of references to "one that wants to give everyone free stuff" or something of that nature.
The little they've mentioned Bernie is nothing compared to the attacks Hillary has received over the past 25 years. I'm much more confident with Hillary's GE support being a floor, because there isn't a negative thing you can say about Hillary that hasn't been said a million times. Bernie has good policy ideas, but so far he hasn't been an impressive debater. Hillary seems to be able to think on her feet quite a bit more, and I'm confident she would wipe the floor with the GOP nominee in the debates.
If Sanders wins the primary, the real barrage will start, and it won't be limited to his socialism. No Democrat would go after his out-of-wedlock child because it's a low blow, but low blows are the Republicans' bread and butter. And yes, sadly, it'll work on some of the voters that Bernie is trying to attract.