Sanders’s early success, like the unstoppable train of Trump’s candidacy four years ago, suggests that the United States has entered a new era of party politics that benefits rebels and ideological radicals. In this era, the most important race is not for the broadest coalition of voters, but for the largest and most passionate base of people who will support the candidate no matter what happens. Instead of encouraging the nominee to compete for the approval of party elites, the primary process rewards whoever arrives at the ballot box with the biggest, loudest base.

David A. Graham: Sanders is winning because he’s popular

The parties once knew how to stop a rebel. In fact, they did it all the time. For most of the past 50 years, Democratic and Republican elites—their party organizations, politicians, cable-news regulars, and various aligned interest groups—successfully used their money and media clout to shape the public’s taste in presidential candidates. This process produced a string of establishment-approved “safe” candidates (Bill Clinton and Bob Dole; John Kerry and George W. Bush) who enjoyed broad elite and popular support. It inspired a 2008 book whose title became a political tagline: The Party Decides.

But that book’s co-author, Martin Cohen, a political-science professor at James Madison University, agrees that today the parties aren’t deciding—at all. “It’s pretty clear that the parties are failing to decide,” Cohen told me. “In the last few decades, the party establishment steered Bob Dole to the nomination in 1996, steered Al Gore to the nomination in 2000, and even steered Mitt Romney to the nomination in 2012. But today’s primaries are harder to steer. You could say the parties can’t even keep the car on the road.”

What changed? Start with the media landscape. In a bygone era of scarcer information, a handful of cable-news channels, radio stations, and print publications set the national agenda. But today, that agenda-setting power has splintered among hundreds of influential podcasts, websites, newsletters, and Twitter and Facebook commentators, all of them building niche audiences that rail against authority. Where once there were gatekeepers, there are now too many gates to keep.

More media mean more direct relationships between candidates and voters. “The sheer amount of media attention doesn’t just make it harder for the party to control the message, but also it makes it easier for candidates to bypass the elites and go directly to voters to raise not just awareness but also lots of money,” Cohen said.

More money means that more candidates are viable. Presidential-campaign spending has soared since the 1990s, and so has the number of candidates in each contest. The 2016 Republican contest featured a libertarian running against evangelical Christians running against classic conservatives—and the winner was a white-nationalist conspiracy theorist who, just years earlier, didn’t even consider himself a Republican. The 2020 Democratic-primary field at one point featured 29 candidates. Today, one could argue that the two candidates with the best chance of winning the nomination are Bernie Sanders and Michael Bloomberg—neither of whom was a Democrat 15 years ago.