In addition to his family’s money, however, Kennedy had one great asset: himself. Before he formally launched his candidacy, Kennedy spent nearly five years developing a national campaign. Through harnessing volunteers, consolidating his local political base, delivering speeches around the country, publishing a hit book and magazine articles and making television appearances, Kennedy did everything he could to build, in today’s terms, a powerful personal brand. He attained a level of celebrity matched by few Americans, inside or outside of politics. He put it to the test in a series of primaries. And he wielded his victories as a giant magnet, pulling old-guard Democrats looking to support a winner into line. By the time JFK gave his acceptance speech calling on citizens to face the challenges of the “New Frontier” at the 1960 Democratic convention, he had already kicked the struts out from a power structure that didn’t realize how close it was to collapse. Kennedy forever changed the presidential nominating system—probably his least-understood significant political accomplishment.

Afterward, as American politics accelerated its shift toward equating legitimacy with popularity, primaries and caucuses effectively took the place of successive rounds of convention ballots. Establishment favorites, like Walter Mondale in 1984 or Mitt Romney in 2012—or Joe Biden this year—would have to slug it out in the field with insurgent opponents to demonstrate they deserved nomination. When competitors offered starkly different visions for their party, as George W. Bush and John McCain did in 2000, it was voters who would decide which prevailed. Relatively little-known but talented candidates, like Bill Clinton in 1992 or Barack Obama in 2008, could build name recognition and support through early-state wins.

And with the brakes off, parties would follow their bandwagons, wherever they led.