All the GOP guns are blazing — at each other. Mutual destruction makes it hard to win an election, and easy to lose one. Just ask President Mitt Romney. The brutal attacks he endured in the 2012 primary softened him up for the Obama juggernaut in the general election. Obama’s claims that Romney was a greedy capitalist who destroyed middle-class jobs echoed charges made first by Newt Gingrich and other Republicans. Trump is the master of insult this time, and takes pleasure in slashing his rivals. Truth be told, he’s good at it, with his mocking of Jeb Bush as...