More Polarization

In addition to the vast democratic majorities in the "good old days"  (50s-60s) the United States faced an existential enemy, a communist, nuclear armed, superpower bent on world domination.  "We will bury you" quoth Nikita Khrushchev.  Communism is about as left as politics can get.  Communism stood for ownership of the means of production (companies) by the state.  In practice that meant liquidation of company shareholders, owners, and for good measure anyone else who stood in the way. 
   This was opposed by all the targeted groups in the US, and by American labor.  Coming out of the 1930's, American workers had organized strong unions and collectively bargained some pretty decent contracts.  The rank and file figured that they had things under control, and any move toward state ownership of the means of production would break their contracts and leave them worse off.  The Democratic party was (and perhaps still is) the party of organized labor,  the Republicans were (and still are) the party of businessmen, who were as hostile to communism as the union men were.
   Which meant that the lefty impulses in the body politic could not move too far from center if they wanted to win an election.  In fact both Democrats and Republicans supported the Cold War, with such effect that the existential enemy suffered political collapse by 1991.
    Now, with communism being pretty dead for the last 25 years,  it becomes possible for American lefties to say things and do things that would have gotten them tossed out of the party in the old days.  "You want to share the wealth don't you," would have branded Obama as a commie back in the day.   In short, the fall of Soviet Communism allows the modern US democrats to go way farther to the left than would have been possible during the Cold War, when communist sympathizers were branded as traitors.  The Republicans have pretty much stayed in place, ideologically.  You don't hear any modern Republican going farther right than good old Barry Goldwater did in 1964. 
   In short, defeat of Soviet Communism has allowed the lefties out of the box and into the body politic where they attract flak.