There are certainly centrist on each side which mirrors a large portion of Americans who lie in the middle ground. It is difficult to see a left that has moved at all particularly looking at FDR and LBJ in terms of legislative accomplishments.
And there are a bunch of conservative Democrats now like there were there then modified radicalism, right?
And there a bunch of liberal Republicans that a President has to work with even to get something going, right?
That's what I'm talking about.
Let me also point this out: the primary process that really went full nonsense in 1972 has caused much of this. Almost definition the activists who get involved in party politics are "true believers" that have a specific goal in mind whether outlawing abortion or free college. These are the people who get revved up and spend their lives working on this and they control the nomination process to a large degree. The Democrats suffered first in this thing by barfing up McGovern and Mondale and then fleeing to Dukakis out of fear of Jesse Jackson winning it. (Carter won because eight liberals split that vote and conservatives/moderates who voted in 1976 and have shrunk to almost nothing voted for Carter out of fear of George Wallace winning it). The Republicans tended to be more authoritarian ("it's so and so's turn") and didn't divide their votes proportionally in the primaries in most states as the Democrats did.
My point is that the rest of us who aren't party members of either party (unless you're in an open primary state) are stuck with whoever the activists barf up for us.
And most of the time the centrists in the party get whacked by the ranting lunatics in the party. It's more prevalent in recent years with GOP dogmatism, no argument there, but Blanche Lincoln suffered the same fate in Arkansas in 2010. The unions didn't like her vote (but never mind that Arkansas had the lowest union membership in the USA) so they targeted her in a summer primary. She won, but they bloodied her so badly she got creamed. I live in Ark when she was elected and let me tell ya - she was a female Bill Clinton in terms of convincing you she was in agreement with your side (whichever one that was) by not actually answering the question you asked.
Anyone who makes the "mistake" nowadays of trying to do something with the other party is going to get whacked in a primary. That is the direct result of the loss of centrist governing in both parties.
An admittedly superficial look would say that with the Clinton's we began a rightward drift that was picked up and weaponized by Dick Cheney to an entirely new virulent strain of right wing extremism that we see in evidence today. You are either for us or our bitter enemy that forces people to move far right to make any compromise at all. Case in point the approach Trump is taking to demand his border wall!
Bush weaponized that in the fallout of 9/11, and I was screaming bloody murder about it then.
(I've never been convinced Dick Cheney was near the "man behind the curtain" a lot of folks want to say he was. Cheney was one of the most popular guys in both parties going back to when he was Ford's chief of staff).
And FTR, the Clintons didn't "start" what's going on now any more than Lee Atwater "started" gutter politics. They all benefited from it, but none began it.