This has been in the cards ever since the hardcore Left took over the core of the Democratic Party. The ouster of Brendan Eich should have caused a great deal more pause than it did. You cannot simultaneously complain about McCarthyism and hound people out of their jobs for their privately-expressed political opinions without consequences. And the consequences are clear: if you tell approximately half of Americans that they are not merely wrong, but evilly so, and in serious jeopardy of losing their livelihood for agreeing with something in 2008 that even the victorious Democratic candidate for President nominally opposed, then you really have shot your wad against the people that you really should be trying to convince.
Message: disagree with the party line for even a short time, even if your stance matches the nominal stance of the party's Presidential nominee at that time, and you're just a tick off Hitler. Well, shit, if you tell people that, don't be surprised when the response is a resounding FUCK YOU.
A few of the more insightful lefties have penned pieces much like this: if you don't want Donald Trump throwing landmines, then don't call Mitt Romney a KKK'er. Call him a bad choice, a misguided choice, a guy who just doesn't know how to make this country the best it can be. Tear him down over and over again on policy, on opinions, on his potential impact on the Supreme Court. But don't say the guy is a racist bigot who wants to reinstate slavery, because once you do that to the biggest milquetoast around, it ceases to have any effect. Lots of people like him, and guess what? Once you've condemned them all as the scum of the earth for having the audacity to disagree with you, with the most generic Eisenhower-era liberal persona that I've seen since GHWB in a Republican candidate, they have lost any reason to try to seek compromise, because you have rejected it in toto.
Obama was a savvy enough politician to skirt his "bitter clingers" remarks; he didn't need those voters to win. Hillary needed some of them. Cf. the Tom Hanks "Black Jeopardy" skit.