I think that's been a proven case for European parliamentary democracies giving way to right wing and fascist parties. But that's more the reverse: the liberal and center-left move center and then lose.
But US politics doesn't have much leftist policy to shed, or many leftist politicians to exoricate, so it is more that even when Democrats win, policy stays right of center. Which is where the frustrated remark "both parties are the same" comes from, which everyone also shits on.
Since you know, the centre right continually stretches further right like a rubber band.