The last four presidential nomination contests have been defined by opportunity and celebrity. The 2020 Democratic nomination race seems unlikely to be an exception.

Opportunity has existed in two forms across these four elections, but was similarly rooted in the negative partisanship that defines American politics. In 2004 and 2012, the incumbent presidents (George W. Bush and Barack Obama Barack Hussein ObamaThe Memo: Trump's strengths complicate election picture Obama shares phone number to find out how Americans are planning to vote Democrats' troubling adventure in a 'Wonderland' without 'rule of law' MORE, respectively) were unpopular with opposition partisans and perceived as vulnerable heading into the election. This led to large fields of challengers with no clear favorites. While both parties eventually settled on “safe” nominees — Sen. John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE (D-Mass.) and former Gov. Mitt Romney Willard (Mitt) Mitt RomneyBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power The Memo: Trump's strengths complicate election picture MORE (R-Mass.) — there were several “lead” changes in the horse race polling during the nomination phase.

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For example, in April 2003, former vice presidential nominee and Sen. Joe Lieberman (D-Conn.) was leading both Kerry and Rep. Dick Gephardt (D-Mo.), but by October 2003, Gen. Wesley Clark was leading Gephardt, Lieberman and former Gov. Howard Dean (D-Vt.). Kerry was still in the top five, but he had dropped from second to fifth. But once the contests got under way and Kerry won Iowa and New Hampshire, activists who were eager to start the campaign against Bush fell in line with the media momentum and made Kerry the winner.

Although Romney’s path to the nomination was harder than Kerry’s because of the newly organized Tea Party, the shape of the race was defined by these activists’ ardent desire to oust Obama from the Oval Office. In 2011 and the first part of 2012, they tried on a variety of candidates from businessman Herman Cain Herman CainThe Hill's 12:30 Report - Presented by Facebook - Big 10 votes to resume football season Exclusive: Internal documents show officials waved red flags before Trump's Tulsa rally Herman Cain account tweets coronavirus 'not as deadly' as claimed after his death from COVID-19 MORE and former House Speaker Newt Gingrich Newton (Newt) Leroy GingrichMORE (R-Ga.) to former Sen. Rick Santorum (R-Pa.). But Romney, like Kerry, had held steady in the top tier and, after he won Florida in late January and secured most of the available delegates in February and March, his opponents dropped out and Republicans closed ranks.

With no incumbent on the ballot, 2008 and 2016 were open-seat elections. This seems like a different kind of opportunity — a race that has the potential to turn into a choice, rather than a referendum. But with the prevalence of negative partisanship, no contest is a choice. All are about enmity. In other words, while some may believe that Obama won his nomination because of his positive message of “hope,” his victory originated in the desire of activists to “change” from a Democratic Party led by the Clintons. Similarly, President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE offered conservative activists a way out from the Bush family’s dominance of the Republican Party and the establishment elites, as signified by the late Sen. John McCain John Sidney McCainThe Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' Cindy McCain: Trump allegedly calling war dead 'losers' was 'pretty much' last straw before Biden endorsement MORE of Arizona.

Celebrity also showed up in two distinct ways. In 2004 and 2012, momentum substituted for celebrity in the nomination contests, but as was revealed in the general election races, momentum fades. The enthusiasm for Kerry and Romney was not lasting, and the anti-incumbent antipathy among independents was not strong enough to prevail. But in 2008 and 2016, enamored activists kept faith with their celebrity nominees and independents were dissatisfied enough to turn these elections into referendums against the incumbent party’s nominees.

So, how will opportunity and celebrity play in 2020? We already know that President Trump’s historic weakness in the polls and high disapproval among opposition partisans and independents has led to the largest field of Democrats ever running.

It is also the case that while former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE may seem like a “safe choice” to take on Trump, Democrats should think twice about what it would mean to nominate someone who is considering a campaign theme of “continuance” as opposed to “change.”

Celebrities don’t have to come from Hollywood, but they do need to have some buzz about them. In order to win, they should already be in the top tier of the horse race polls, be raising solid sums of money, and have a shot at victory in at least two of the first four contests.

When looking across the 2020 candidates, the ones that fit this celebrity bill are Sen. Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.), former Rep. Beto O’Rourke (D-Texas) and, somewhat amazingly, Mayor Pete Buttigieg Pete ButtigiegThe Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice Hillicon Valley: FBI, DHS warn that foreign hackers will likely spread disinformation around election results | Social media platforms put muscle into National Voter Registration Day | Trump to meet with Republican state officials on tech liability shield MORE of South Bend, Ind. Even though Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) also is in a position to win the contest, it seems there are too many in the party who view him unfavorably for his celebrity to unite the activists around his candidacy.

Looking ahead, we’re surely in for a wild ride, fueled by a debate between those Democrats who believe the party should choose “nostalgia” and a “safe choice” (Biden and the Midwestern battleground) as opposed to “hope and change” and the “future” (Harris, Beto or Buttigieg and the Southern battleground).

But if there is anything to learn from the last four contests, it’s that when activists fall in love with a celebrity candidate, they elect them. Simply put, Democrats need a celebrity candidate to beat Trump — and Biden is too-much “yesterday’s news” and a “turn-off” to those who voted against Obama’s legacy in 2016 to be that person.

Lara M. Brown is director of the Graduate School of Political Management at George Washington University. Follow her on Twitter @LaraMBrownPhD.