It’s the candidates who play the long game, and play by the establishment’s rules, who usually win presidential nominations. Political parties have lots of ways to influence the race in favor of these candidates, from how they appoint superdelegates to how they schedule debates. Hundreds of millions of dollars are spent on advertising, meanwhile, and the bulk usually favors establishment candidates. And voters have a lot of time to make their decisions and can amend them as they go along — an insurgent candidate who wins Iowa or New Hampshire won’t necessarily have staying power if they’ve failed to build a broad coalition of support.
The short run is different. The short run can be crazy. Feedback loops can produce self-reinforcing (but usually temporary) booms and busts of support. For instance, a candidate who has some initial spark of success, such as by doing well in a debate, can receive more favorable media coverage. That, in turn, can beget more success as voters jump on the bandwagon and his poll numbers go up further.
Candidates can just as easily get caught — or entrap themselves — in self-reinforcing cycles of negative media attention and declining poll numbers. Hillary Clinton looks like she’s stuck in one of these ruts right now.