this has not been my observation
I guess it depends where you live.
Aside from the gay marriage issue and the decline in traditional religion, I'd say Americans in many ways are more conservative than in the past, especially the 70's.
That's an Empirical question, hard data would be nice. But using the 70's as some gold standard isn't really fair. That's almost like like saying Germany took a leftward swing after world War 2, or the Soviet Bloc went right after its collapse. That was the most left wing Era the US had.
Besides the baby boomers are in full power now, and our generation has been raised off their stories. Any of us who are artists, live in a left wing city, attended the humanities,or just watch TV ought understand this.http://www.thedailybeast.com/articles/2013/09/12/the-rise-of-the-new-new-left.html
It may be fair to say we are the most left wing since the 70s .... but still less so than the 70's and 30s.