If Republicans are going to have any hope of stopping the runaway train otherwise known as Donald Trump, it may rest on their ability to limit the breadth of his appeal. A single alternative to Trump — most likely Marco Rubio — could conceivably coalesce enough support among well educated, relatively affluent, more moderate Republicans or more mainstream conservatives and go on to triumph in winner-take-all March 15th states like Ohio and Florida, forcing a delegate showdown at the convention for the nomination. Super Tuesday looms as a big test of this proposition. As Ron Brownstein puts it: If Trump...