“The press is your enemy,” said the president. “Enemies. Understand that? ... Because they're trying to stick the knife right in our groin.” Donald Trump's ongoing feud with the media is not the first time a president has felt wronged by the press. The opening words are those of Richard...

What's the best predictor of which candidate will win the presidential nomination? The winner of the Iowa caucus? The winner of the New Hampshire primary? Actually neither is as good an indicator as the winner of what political scientists call “the invisible primary” — the period before a...