With respect to the admirable Walter Karp, while there were certainly members of the (former) Democratic establishment who wanted to sabotage McGovern’s chances, they were aberrations. John Connally had a vanity project, Democrats for Nixon, which claimed a large following but really was just a few spoiled children. Likewise the Chicago machine was an explicit ‘victim’ of the post-1968 changes, so I can well believe that a ward heeler (as quoted) would have vindictive tendencies.
We have to keep reminding ourselves that people like Connally (in 72) and, say, Rahm Emanuel (today) are not the Democratic establishment; they are self-important blowhards trying to pretend that the system hasn’t moved on and left them behind.