For those of you who are too young to remember the George McGovern campaign of 1972, AFTER got the nomination, there was a full court press to ensure that he lost the general election from his own party, who anonymously peddled the whole, "Acid, Amnesty, and Abortion," meme to people like ratfuck Robert Novak.
This compares to the dissatisfaction of the left in 2010 about, for example, the lack of a public option, or how the HAMP program seemed more for banks than homeowners, where dissatisfaction was pretty much limited to griping on places like Daily Kos. (And for which "the left" was pilloried)
The Bob Rubin wing of the Democratic Party will go postal on any presidential nominee who seems serious about addressing inequality, reining in Wall Street, opposing "free trade" deals that favor capital over labor, and fighting the military-industrial complex.