It's a familiar plotline. An interloper runs for a party's presidential nomination and, with an anti-insider pitch, scores wins and near-wins in the first contests with vote pluralities. His numerous opponents, fearful of antagonizing his enthusiastic supporters, launch attacks on one another that, predictably, hurt the attacker as well as the target. Party establishment types, convinced the interloper is a sure loser in November, dither and tilt things mildly against him while trying to maintain the impression of fairness. Candidates with no chance remain in the race, dividing the anti-interloper vote, hoping the interloper will collapse. But he doesn't, and...