It helps that I was alive at the time, and thus was able to read this stuff in the newspaper, not the history books or websites. I don't have that first hand experience of FDR, and I've read that some of the Republicans he had to deal with were pretty hostile. But the Republicans were roundly trounced by FDR in 1936 (at that time the worst presidential defeat in American history), and after that some level of support for the FDR New Deal was essential for their survival. Even Nixon was way to the left of Clinton on many issues, having founded the Environmental Protection agency, and the National Endowment for the Arts. Then Reagan got elected, and pushed the country further to the right than it had been in decades.

If you look at the bills that Biden is trying to pass, you can see that he is the most leftist president since FDR. He's old enough to remember FDR, and is trying to keep that ideal alive today. People say he "failed to keep his promises" because the Republicans refuse to let him pass anything. But the bills he is trying to pass, and the few he succeeded in passing, are often the same bills that Warren and Sanders would have proposed if they had been elected. And his years in congress probably gave him a better track record at passing bills than any of the so-called leftist candidates would have acheived.