It seems likely that the adrenaline that has accompanied the Democratic Party’s brush with political death is doing a lot of the work of generating enthusiasm for the party’s new presidential prospects for both Kamala Harris and Tim Walz. But one thing you can say in Walz’s favor, though, is that his demeanor is that of a happy warrior, and his attempts at humor have a natural and unrehearsed feel. As a performer, Walz has skills. But his material should leave his supporters apprehensive...