At its least sectarian and dogmatic (pre-1970), the idea of the Left and of much of rock music and associated culture was connected, typified by people like Lennon and Dylan.
mbs
New Left
I don't think the New Left and the counterculture merged all that much. Yeah, Country Joe and the Dead played rallies, but wasn't it the Panther, SDS (and later Weather) critique that the counterculture was decadent and frivolous? By the late-60s and early 70s, much of the New Left had hardened into a dogmatic and at times violent culture, while the counterculture began receiving corporate bucks to entertain the new consumers advertisers coveted.
Even in the 80s, when I attended cultural events tied to anti-nuke or Central American issues, there was usually an earnestness that drowned out the fun that we were supposed to have. I'd find more spontaneous human expression and happiness at the block parties in Alphabet City and parts of Brooklyn than I'd see at events organized by white lefties, where people tried to give you leaflets while you danced.
DP