Everybody and their brother, since 1945, has been claiming some epic take-over of the Democratic Party. There’s probably fifty different groups declaring their ownership and command of the Democratic Party.
If the gameplan for the Republican Convention is some BLM maneuver....at least a quarter of the Democratic Party will vacate permanently and become firmly attached to Trump.
I don't know about some epic takeover but just 4 years later the Democrat nominee was George McGovern, so yes 1968 was the beginning of the Leftist takeover, there is no way McGovern would have been nominated in a sane would. They had been toying with it from the FDR days but went over the cliff in 72.