Is the bulging field of Democrats seeking the 2020 presidential nomination a sign of strength? Or is it a troubling indication that wresting the presidency from Donald Trump won’t be as easy as it ought to be?

At last count there were 23 “major” candidates including seven senators, four members of Congress, three mayors, two governors and a clutch of other hopefuls. The field has something for everyone: young, old, male, female, black, white, Latino, Asian, gay and straight. The...