The party strikes back: How Biden's prospects soared so quickly

This era can often seem like one of political chaos. Renegade candidates win elections while brushing aside their own party elites. One take-away this spring, however, is that political parties aren't dead.

Two weeks ago, Joe Biden’s presidential candidacy was almost dead. Today he looks like the presumptive nominee of the Democratic Party. What happened?

In part it was geographic. The former vice president’s core supporters were scarce in early voting states, but more prevalent from South Carolina on. He also likely benefited from a bandwagon effect, attracting support...