These three examples have a common theme. Late candidacies, almost by definition, begin with enormous hype. If that hype isn’t instantly met with an exceptional performance, the poll bubble bursts. There’s no time to iron out kinks and shake off rust. There’s no margin for error.
Candidates that start earlier, in contrast, have time to overcome mistakes and turn the page on past embarrassments. In the current presidential primary, Elizabeth Warren apologized for identifying herself as an “American Indian” back in February. At the time it seemed like the lingering controversy about her ancestry was going to stifle her campaign before it could get off the ground. Today, several white papers later, the matter rarely comes up (at least, among Democrats). Similarly, Joe Biden has been able to move past his controversial comments in June about working alongside segregationist senators during his early career.
Party loyalists can get anxious around the midway point of the primary season because presidential candidates are flawed human beings, and the harsh spotlight of the campaign trail exposes those imperfections. Even today, when the Democratic leaders all consistently beat President Trump in trial-heat polls, seeing those candidates perform inconsistently on the stump, take policy positions that carry general election risk, or awkwardly shift on issues out of political calculation, stirs up worries that the polls will shift for the worse in the fall of next year. Candidates on the sidelines, who have yet to go through the wringer, will always look shiny by comparison … until they get off the sidelines.