CBS News poll analysis: Some Democrats don't want Biden to run again. Why not?

First off, a majority of Democrats do think Joe Biden ought to run for reelection, which he is doing.

But it's not a huge majority; more than a third of Democrats don't think he should run. And some Democrats have been voicing this doubt for a while.

So, key questions remain: What exactly are their concerns? And is that third motivated by a feeling that he's done a good job but now is the time to move on, or is it mixed with any discontent with his performance?