If Clinton thought Bernie was a mean old man, get ready for Trump to just destroy her. He has no filter and will eviscerate the Clinton name and bring up her shady history in politics.
Worth saying that I doubt she
actually thought Bernie was being all that mean. Because he wasn't, relative to the standard. Instead, it was standard campaign tactics: attack your opponent's strength. A big part of Bernie's appeal was his ethics and integrity and "I have always been consistent in my beliefs". He spent the first half of the campaign talking all about how he's never run a negative campaign; he runs positive campaigns. Bernie being a good honest guy, though kinda grumpy, and all that stuff went into the persona he was building up.
Then he went negative, and so she called him on it. If an oil executive or hedge fund manager donated to his campaign, then she would've turned that into a story. Take a guy who talks about integrity, and make him look like a hypocrite. Make people think "man, this is not the guy I supported a few months ago". Whether you think she did a good job of that or not is another story, but she won in the end so whatever.
It's also kindof a labeling thing, which is a great campaign tactic if you do it right. Obama did it fantastically in 2012. Trump has also shown a knack for this. Lyin' Ted, Little Marco, Low Energy Jeb. And his unparalleled dominance of the news media made his side the only one that got out. Like, he fucks up plenty (that time he had like five positions on abortion over the course of two days. This current judge bullshit), but he's got a talent for just shitting on people.
This was, again, relatively weak as far as negative campaigning goes. The Hillary campaign didn't even bother diving through any skeletons in Bernie's closet (they exist). Trump is not pulling any punches, so I imagine that the Clinton campaign is gonna have to take the gloves off too.
Personally, I can't wait for Obama to get involved this fall. I'm sure he's dying to get some payback on Trump.
