FDR was moved to adopt left wing ideas on account of the pressure that was mounted on his administration from the left. Specifically the very important socialist and communist groups that existed at the time.
"After his election in 1932, FDR met with Sidney Hillman and other labor leaders, many of them active Socialists with whom he had worked over the past decade or more. Hillman and his allies arrived with plans they wanted the new President to implement. Roosevelt told them: "I agree with you, I want to do it, now make me do it."
I would love to see a new Leftist Movement arise and gain sufficient power that they could demand change and actually get it. It's going to happen, it's already begun. But as much as I wanted to, I don't think I'll live to see it. You probably will, Chunk, you're younger and healthier than me. I think it's going to take most of a generation, about 30 years, to pull it off.
But it HAS begun. "Medicare For All" has become a thing. It isn't going to just go away. We're a few election cycles away from it being discussed on the floor of the house but I think it's inevitable.
Living wages are also just visible on the horizon and I expect something to be done about higher education soon.
If we accomplish those three things then I think The USA will have a chance at becoming great for the first time in its history.