In early July, during another rough patch for Hillary Clinton’s presidential campaign, Dan Pfeiffer took to CNN to reassure his party. Pfeiffer used to be President Obama’s top communications aide. The title of his op-ed was “Stop the bed-wetting: Hillary Clinton’s doing fine.” Bed-wetting, Pfeiffer explained, “is a term of art in Obamaland.” Ah, the president and his acolytes. Such sophisticates. Clinton shouldn’t panic, Pfeiffer argued, because she remains ahead in polling and in fundraising, because Bernie Sanders “is not Barack Obama,” and because “Hillary Clinton circa 2015 is not Hillary Clinton circa 2008.” Elections, after all, “are about fundamentals,”...