Two nights. Twenty candidates. The first Democratic debates mark a new phase in the presidential race

The stage at the Adrienne Arsht Center for the Performing Arts on the eve of the first Democratic presidential debate of the 2020 campaign.Doug Mills/New York Times

MIAMI — In a windowless conference room in downtown Miami, Julian Castro practiced answering mock debate questions while an aide kept an eye on the clock, raising two hands in the air whenever Castro spoke longer than one minute.

Elsewhere in the city this week, another Democratic presidential contender, California Senator Kamala Harris, huddled with her advisers over Caribbean food. Massachusetts Senator Elizabeth Warren squeezed in one last rally with her final debate preparations. And New Jersey Senator Cory Booker’s aides passed around a memo laying out his goals for the mere 7 to 11 minutes he will likely speak onstage.

Most of the Democrats seeking their party’s nomination are preparing to meet in Miami on their biggest stage yet beginning Wednesday, for a two-night, 20-candidate extravaganza that marks a new phase in the sprawling campaign to take on President Trump. The events will put household names like former vice president Joe Biden on the same stage as little-known Democrats like Andrew Yang, offering leading contenders a chance to prove their polling leads are about more than name recognition while giving upstart candidates an opening to grab the nation’s attention.

But instead of wrestling with substantial questions about the future of the nation, the event will probably feel like speed-dating, with 10 candidates standing elbow-to-elbow each night, all trying to pack soaring rhetoric or clever policy prescriptions into the 60 seconds they have each been allotted to answer questions by the organizers of the debate.

“It’s going to be such a short time to get to know them,” sighed Donna West, the chair of Nevada’s Clark County Democrats. “I imagine most of the candidates are feeling enormous pressure.”

The candidates will have no opening statements, according to NBC News, which is hosting the first debates along with MSNBC and Telemundo, and they will get 30 seconds for follow-ups in addition to the minute they get to answer questions from the event’s five moderators.

“It’s not a debate, it’s a joint press conference,” said Jeffrey Berry, a political science professor at Tufts University, who said the forums could nevertheless play a crucial role in shaping the unwieldy 2020 field, particularly as little-known candidates like Representatives Eric Swalwell or Tim Ryan seek a breakout moment.

“This could be really a do-or-die moment for some of these less-visible candidates,” Berry said.

It all kicks off Wednesday night, when Warren will take the stage alongside Castro, Booker, Senator Amy Klobuchar of Minnesota, former Texas representative Beto O’Rourke, and five other candidates. Warren has been climbing in the polls, but all of the other contenders have been mostly mired in the single digits.

Thursday night’s debate is widely seen as the major clash for top-tier contenders. That night, Biden, Vermont Senator Bernie Sanders, Harris, and the insurgent South Bend Mayor Pete Buttigieg face off with six more candidates.

It will give most Democratic voters their first chance to compare candidates side by side as they consider which one seems best-positioned to take on the president.

“Who looks like they are strong enough, tough enough, serious enough to both be president and defeat President Trump?” said Neera Tanden, the president of the liberal Center For American Progress, who is not aligned with a candidate, describing the central question for many voters.

But as candidates from different wings of the party come together onstage for the first time, the debate could open the door to spats and attacks that some Democrats worry will bruise the eventual nominee. “Democrats are very anxious about not having a candidate, the nominee, bloodied,” Tanden said. “Any candidate that’s attacking has a risk.”

Indeed, the debate is coming as some of the early divisions in the crowded primary field are beginning to reveal themselves. Sanders, the Vermont progressive, last week sent a tweet that appeared to knock the populist Warren for drawing support from some centrist Democrats, and the two may seek to curry favor with each other’s supporters, even though they are set to appear on different nights.

Also last week, Biden, who has been leading national and early state polls since he entered the race in April, drew disapproval and condemnation from some of his opponents, particularly Booker, for waxing nostalgic about his working relationships with segregationist senators. That could make Biden an inviting target for criticism or veiled attacks.

In 2016, it was Republicans who had droves of candidates running for president, and the crowded debates shaped the race. Trump benefited from his ability to dominate the stage, and candidates perceived as front-runners, like former governors Scott Walker and Jeb Bush, seemed to pale in comparison. Florida Senator Marco Rubio was memorably eviscerated on a crowded stage in New Hampshire by New Jersey Governor Chris Christie.

“If you lose an exchange on that stage, your campaign may be over,” said Alex Conant, who was Rubio’s communications director that year. On a crowded stage, candidates need to flout the ground rules when necessary, he said.

“It’s easy to be forgotten on the stage, so you need to be aggressive about interjecting, and being part of the discussion,” Conant said.

This time, candidates will face different tests onstage depending on how widely they are known. Those who are polling in the low single digits, like the former Housing and Urban Development secretary Castro, will need to make a good first impression on voters who barely know who they are.

“For a candidate like Julian where his name ID might not be as high as others, this is an audience of potentially 25 million people who are watching him for the first time,” said Sawyer Hackett, an aide to Castro. “It’s a matter of whether you can condense your biography, your experience and your plans.”

Warren, who is by far the most prominent candidate in Wednesday night’s matchup, is likely to attract the most scrutiny and attention, and she may use the occasion to try to salve lingering reservations from some in the party about her ability to take on Trump.

“What she needs to communicate is that she’s presidential, that she can translate her ideas into action,” Berry said. “Elizabeth Warren needs to be the most dynamic person in the room.”

Warren’s supporters hope her rhetorical skills — honed over years of teaching and debate as a teenager — will help cement her status in the race’s top tier, especially because she won’t have to fight off jabs from Sanders or Biden.

“She’ll get all the spotlight of a presidential debate stage and the ability to get her big-picture thinking across without the tensions of squabbling with other front-runners,” said Adam Green, the cofounder of the Progressive Change Campaign Committee, which supports Warren.