> To be left-leaning in the 60s and 70s was to be alternative to the mainstream. Today, to be left leaning is to be mainstream.

Not really. It's true that a position left of the center established by the balance of the parties in government is mainstream in the population, but that's because the parties in government have moved right relative to the population (the Republican party consistently, the Democratic Party merely on average as a result of the big rightward jump of the Clintonian "third way", from which there has been some subsequent minor movement back toward the left, with some ambiguous signalling that bigger moves in that direction are possible in the near future.)