Live television coverage of the national party conventions gives the nominees the opportunity to present themselves unmediated and at length to the American people. There's no moment in the presidential campaign when the candidates so fully control the message as during the party conventions....

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...