cnn democratic presidential debate
What it will take to break through in presidential debates
04:53 - Source: CNN

Editor’s Note: Julian Zelizer is a professor of history and public affairs at Princeton University and author, with Kevin Kruse, of the new book “Fault Lines: A History of the United States Since 1974.” Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion at CNN.

CNN  — 

The stakes are high as 20 Democratic presidential candidates get ready to meet in Detroit for their second round of televised debates. Given that the first debates produced some sizable short-term shifts in the polling after Sen. Kamala Harris’ direct challenge to frontrunner and former Vice President Joe Biden on school busing, everyone is aware that the things that are said on the debate stage can make a big difference.

While the dynamics of any debate are impossible to predict, each candidate has a specific set of objectives to best position themselves as the campaign moves ahead.

Joe Biden

The former Vice President needs to show in front of the cameras that he is in fact the most “electable” candidate. The basic premise of his campaign is that he is best positioned to defeat President Donald Trump. With Democrats desperate for a new person in the White House, each poll showing Biden’s strength against Trump offers more fodder for the campaign.

Yet the first debate raised some serious questions. Some of the well-known baggage that Biden brings to the table, some of the compromises he has made along the way in his Senate career and his tendency to stumble in high-profile moments, all came to light within a few seconds.

The debate in Detroit offers him another chance to prove to voters that he can perform in this arena. Democrats need to imagine what he will look like when facing off against Trump during the crucial months of next year’s general election. At this point, Biden’s biggest vulnerability is his tendency for self-inflicted wounds during the campaign.

Elizabeth Warren

By many accounts, Sen. Elizabeth Warren has run the best campaign of any of the candidates. She has risen in the polls by offering a sophisticated mix of stump speeches, policy proclamations and social media blasts. She has also taken a series of bold stands, such as calling for impeachment early on and favoring ending the Senate filibuster, that distinguish her from the rest.

But given Biden’s standing she will need to consolidate the non-Biden Democrats, along with the marginal Biden supporters, around her candidacy. This means that in the debate her objective will be to keep making the case that she is the most credible coalition builder rather than the frontrunner.

She has to show that she has the capacity, including on foreign policy issues, to hold and excite progressive voters while still appealing to the mainstream of the party. In other words, she has to show that she is the candidate most likely to rebuild the Barack Obama coalition that guided the party to victory in 2008 and 2012. Going head-to-head with Bernie Sanders offers her an opportunity to make this case.

Bernie Sanders

Sen. Bernie Sanders has remained steady, but he needs to show that he can broaden his support. He still commands a passionate following. For those Democrats, he remains the most authentic progressive voice and a politician who has been brave enough to be bold on important issues.

But his support has not greatly expanded and in some places it has diminished. Sanders needs to use this debate to ignite the crowd. He needs to convince Democrats outside his orbit that his policy positions are winning issues and that he can best appeal to the working-class voters in states like Wisconsin that Democrats desperately want to win back. He needs to go beyond repeating his accomplishments from 2016 and make a case for 2020.

Most important, Sanders needs to show that his being at the top of the ticket would not open up the party to defeat as President Trump unleashes his “Democrats are socialists” campaign. If he was the candidate, some wonder, would Democrats lose voters to Trump in an even broader range of states than the last time around?

Kamala Harris

Sen. Kamala Harris has shown repeatedly that she is incredibly powerful on the public stage. During congressional hearings and in the first debate, Harris demonstrated numerous times that she can handle tough questions and knows how to eviscerate an opponent with remarkable rhetorical precision. Without going low, she can hit hard. But what is her campaign about?

Harris still remains something of a mystery. While Biden is all about the promise of being electable, much of Harris’ buzz remains the fact she is a smart and skillful politician. But she needs to fill in some of the gaps by talking about the issues that matter most to her and outlining a bolder agenda that she seeks to pursue. As someone who has spent much of her career as a pragmatist, offering such a vision may not be easy.

Even though she is not on the stage with Warren, she needs to offer an argument about why her candidacy is more powerful than the other major non-Biden frontrunner. This is her chance to introduce the heart of her run.

Pete Buttigieg

Mayor Pete Buttigieg’s poll numbers remain low but his promise remains strong. His fundraising haul has been impressive and he has stepped up his campaign with policy agendas on issues such as criminal justice and the economy. The 37-year-old mayor is smart and politically savvy but still needs to overcome concerns about his young age and relative lack of experience, as well as whether he is too moderate to win over progressives.

While the mayor brings many strengths to the table and remains in strong position to win over Biden voters should that campaign falter, he has yet to really sell the most powerful part of his candidacy: generational change. This was a theme that he raised when he announced but one that has somewhat faded ever since. He has to use his age to his advantage and argue that expunging the politics that produced Trumpism requires a candidate from a new generation, one willing to think about problems in fundamentally different ways than older politicians.

He needs to deal head-on with the structures of politics, such as the Electoral College, that keep reproducing dysfunction and made a President like Trump possible. As a well-educated, married gay man who voluntarily served in the military and focused on local politics, his life story is a symbol of the frustration many Americans feel, as well as the Kennedy-like promise of a new era.

Everyone else

Get our free weekly newsletter

  • Sign up for CNN Opinion’s newsletter.
  • Join us on Twitter and Facebook

    At this point many candidates are really hanging on by a thread. Some once-promising candidates like Amy Klobuchar, Beto O’Rourke, Julián Castro and Cory Booker are failing to gain much traction. By the fall there will be pressure for some of them – let alone others such as Seth Moulton, who have barely made a blip – to step aside and make room for the main candidates.

    If they’re not able to make some kind of splash, the pressure for them to exit will intensify. They have to offer a strong taste of what distinguishes them and what makes them viable should there be openings if any of the top candidates falter.