It has happened before, in both political parties. At the beginning of a presidential primary season, with a big field of candidates starting the race, the party faithful say, Isn’t this great? We have so many good candidates — almost an embarrassment of riches! Then, after several months of campaigning, they say, Can’t somebody enter this race and save us?

That is where some Democrats find themselves right now. The party’s top three candidates in national polls are Joe Biden, who is too old to be president; Elizabeth Warren, who is too radical; and Bernie Sanders, who is too old and too radical.