Unfortunately, the president violated one of the cardinal rules of combat: Never interrupt your opponent while he’s making a mistake. Even as Donald Trump competently prosecutes a war abroad, he might have fumbled a sector of the domestic conflict against his Democratic adversaries by weighing in on California’s gubernatorial race.

On Monday morning, Trump endorsed former Fox News personality Steve Hilton’s campaign over his toughest Republican competitor, onetime Riverside County, Calif., sheriff Chad Bianco.

Hilton needed the boost. For months, polling showed that Hilton and Bianco were neck and neck, both drawing support in the low double digits. But as the only ...

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