Jeb Bush's backers have watched as Mr. Bush debuted his White House bid amid chatter of inevitability, then plummeted in polls. They have seen him bungle a debate performance, losing a pivotal exchange to his one-time protege, Senator Marco Rubio. And they have observed as Mr. Bush, in small towns and big cities, has proved a much less gifted campaigner than they ever anticipated. So on Saturday, as many of the Bush faithful descended on the Biltmore Hotel near Miami to hear from Mr. Bush and his team, his aides tried to put their most optimistic spin on a campaign...