Donald Trump has a knack for choosing weak adversaries. He recognized the potential in taking advantage of undocumented workers. He used the element of surprise to take out Vince McMahon. Then he cut through a field of supposedly formidable Republicans as though they were Lilliputians, setting himself up for a general-election match-up against Hillary Clinton—who, it turned out, was a notably weak Democratic nominee. Now it’s the press’s turn. While he’s bashed the news media since early in his campaign, the president has recently elevated his feud, with attacks on Mika Brzezinski and CNN that shocked even benumbed observers. Reporters...