To paraphrase Santayana, Democrats who refuse to acknowledge Hillary Clinton’s failures in the 2016 election were always doomed to repeat them. Why is their primary field littered with the failed bids of woke candidates? Why is #WarrenIsASnake trending on Twitter? Because identity politics remains a political loser.

That’s the takeaway from the rapidly narrowing Democratic field, and smart liberals warned of it after 2016. Mark Lilla, writing in the New York Times, faulted Mrs. Clinton for molding her campaign around “the...