Has Joe Biden Lost His Ability to Unite Democrats?

By this time next week this will be official: Joe Biden is making his third race for the presidency (earlier runs were in 1988 and 2008). In conventional terms, he begins his campaign as the front-runner, or at worst the co-front-runner with fellow mid-septuagenarian Bernie Sanders. He has led in 36 of the 37 polls included in RealClearPolitics’ compilation of 2020 Democratic-nomination-contest surveys. He also leads in all the publicly available polls of Iowa, and even leads in Kamala Harris’s California.

But the big question haunting his candidacy now is...