Sen. Bernie Sanders (I-Vt.) has suspended his race for the Democratic presidential nomination, leaving the establishment-favored former vice president Joe Biden as the party’s presumptive nominee. Over the last five years, Sanders has headed an insurgency within the Democratic electorate, blaming Democratic elites for failing to combat mounting economic inequality.

Many political scientists subscribe to a theory about presidential nominations that was summed up by a 2008 book, “The Party Decides” by political scientists Marty Cohen, David Karol, Hans Noel and John Zaller. First articulated in 2008, the argument goes that party elites coordinate on a preferred candidate from within the ranks of established party figures -- and voters then follow signals sent by the party establishment. According to the theory, candidates who run as outsiders should never gain their party’s nomination.

The theory seemed to collapse upon the nomination of Donald Trump in 2016. But in the face of Sanders’s loss, perhaps the party decides after all?

The years 2016 and 2020 can teach us a lot about how parties work. Let’s take stock.

Parties are contentious institutions

Parties are unique, multi-part institutions. They are organized both inside government, coordinating legislators around a common identity and agenda, and in the electorate, uniting voters and party activists. In other words, parties are both policymakers and vehicles for the popular, partisan will.

But parties’ dual role also creates dilemmas. For U.S. parties in particular, with their loose organizational structures, it is not clear who really governs a party.

On the one hand, officeholders make a plausible claim to rule. They have specialized policy expertise and legislative experience that most voters lack. And they are likely to take a longer view than voters, claiming to know what’s in the public interest.

On the other hand, voters can also be said to govern. In a democracy, voters temporarily delegate power to representatives on Election Day. And what voters want ostensibly shapes parties’ platforms and policy agendas.

This makes parties contentious institutions. Because their internal lines of authority are not well defined, and because both politicians and partisan groups compete for authority, parties inevitably become arenas for competition and conflict over what the party does and what it stands for. That means that intra-party tension -- especially over the selection of the party’s standard-bearer for president -- is par for the course.

The establishment is real, but it does not rule

We don’t need to succumb to conspiracy theories to accept that both parties have genuine “establishments.” As durable organizations, Democrats and Republicans have developed groups of experienced leaders and stakeholders with considerable resources.

Given the winner-take-all nature of U.S. elections, party establishments are likely to be risk averse, eschewing anything that may jeopardize winning. This often puts them at odds with activists seeking to use parties as vehicles for major policy change.

Such tensions were on clear display in both the Trump and the Sanders insurgencies. In 2016 and 2020, scores of establishment figures worried aloud that nominating a polarizing candidate could have devastating effects on their parties.

Yet both establishments encountered considerable difficulty thwarting their insurgents. Trump, of course, ultimately prevailed. And while Sanders did not, he performed better than most expected.

Thus, while the party establishment is real, its success is hardly guaranteed.

Insurgents can storm the gates, but the party can respond

But party elites are not powerless, especially in face of an insurgent threat.

As we saw in the Democratic primaries this winter, when Sanders emerged as the front-runner after Nevada in early February, the party establishment kicked into full gear. Immediately after the South Carolina primary on March 1, Sen. Amy Klobuchar (D-Minn.) and former South Bend, Ind., mayor Pete Buttigieg dropped out of the race and endorsed Biden. According to reports, Buttigieg had discussed his options on a call with former president Barack Obama, who counseled the former mayor to leverage the moment wisely. Klobuchar’s endorsement was likely critical to Biden’s success in Minnesota, where he lacked any serious organization.

But note how publicly and belatedly these pro-Biden efforts occurred. “The party decides” theory says that party elites coordinate among themselves during the year prior to the nominating contests, known as the invisible or money primary. This time, establishment figures waited cautiously during the early primaries, unwilling to rally to Biden as he floundered. Only once Sanders clearly had a path to the nomination did the calculus change. Endorsements from party leaders -- following, rather than leading, developments – arrived quickly. Biden bounced back to outperform expectations in South Carolina’s primary and then, three days later, in most Super Tuesday states.

Party elites may have “decided,” but that decision was influenced by the open contest. The outcome was not inevitable.

Why did the Democrats succeed where the GOP failed?

In thwarting Sanders, Democrats had one major advantage that the GOP didn’t: history. Trump’s insurgency had taught the Democrats how essential it is to close ranks behind an alternative, whatever that person’s shortcomings.

Back in 2016, Republicans waited too long to counter Trump, content to believe he would inevitably implode. By the time it was clear that he was indestructible, Jeb Bush was long gone; the two candidates left standing, Sen. Ted Cruz (Tex.) and Sen. Marco Rubio (Fla.), were unpalatable alternatives.

Further, the Democrats and Republicans have very different kinds of parties, which may have affected their abilities to thwart insurgents. Many say that Republicans speak in the language of ideology while Democrats speak in policy. In 2016, Trump painted the Republican establishment as insufficiently committed to conservative principles, even as he departed from standard GOP positions himself.

Sanders, by contrast, defined his democratic socialism almost entirely in policy terms, and his rivals responded by proposing versions of their own. While his ideology also set him apart, he couldn’t wield it against them, lest he discredit himself.

Democrats and Republicans may have different abilities to fend off insurgent challengers. But their nature as contentious institutions means their success is never guaranteed.

Adam Hilton (@adhilt) is assistant professor of politics at Mount Holyoke College whose book, “True Blues: The Contentious Transformation of the Democratic Party,” will be published by University of Pennsylvania Press in 2021.