The Republican primary, on the other hand, will be all hell bursting loose. The candidates will spend the next year tearing each other apart on everything and anything. Super PACs are furiously raising money, some of which will be used to take down and slam GOP opponents in negative ads and videos. At least a few of them will do what Newt Gingrich so effectively did to Mitt Romney in South Carolina in 2012. Mr. Gingrich hit hard on Mr. Romney’s investment firm, Bain Capital, and his tax returns. He painted Mr. Romney as a cold, rapacious capitalist who’ll close your factory and take your jobs. Mr. Gingrich described Mr. Romney’s line of work as “rich people figuring out clever legal ways to loot a company.” Mr. Romney’s South Carolina numbers began to sink in the last days of the campaign. Mr. Gingrich enjoyed a surprise win.

The Obama re-election campaign was of course watching the fun, and went on to kill Mr. Romney with Mr. Gingrich’s themes. They’d likely have done it anyway but the attacks were given added legitimacy by GOP provenance.

The Democrats have an enforcement mechanism to keep all their candidates in line. Bernie Sanders and Martin O’Malley know without being told that the party will kill them if they tear apart the assumed nominee. Their careers will be over if they go at her personally.

A GOP opposition-research veteran said of the Democrats’ enforcement mechanism, “As an upstairs-downstairs party, the upstairs is a fairly concentrated place. The Democrats as the ‘in’ party—the party of Silicon Valley and academia—has interlocking pools of money, brains and talent.” When they turn on you, it is like facing “the Death Star.” And “on top of that, you have the Clintonian tropism toward score settling and vengeance. What you have in the end is discipline.”