Correction: Updated to correct date of Rick Perry's debate misstep.

Can six minutes of fast talk on a crowded stage ignite a presidential campaign?

Twenty Democratic contenders hope so.

The first chapter of the Democratic presidential contest has had an everybody-in-the-pool spirit: Seven senators and five representatives and two governors and three mayors and even a self-help author announced bids for the nomination to challenge President Donald Trump. A former Pennsylvania congressman just became the 24th hopeful in a field that already set records for size and diversity.

The second chapter of the campaign is being launched at the Adrienne Arsht Center for the Performing Arts in Miami on Wednesday and Thursday in back-to-back debates.

Democratic debate No. 1:What you need to know

The campaigns of most of these contenders are likely to be history by the time the opening Iowa caucuses convene next February. As University of Michigan debate expert Aaron Kall notes, the opening debates will be “integral” in determining who survives until then.

Former Vice President Joe Biden, leading in national and state polls, is preparing to respond to attacks that he represents the party’s past rather than its future, that he is a well-liked figure but one with baggage. Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts are unveiling far-reaching policy proposals on college affordability and taxes as they vie to win the favor of the party's most liberal voters. Pete Buttigieg, a fresh face who has had a fast rise into the top tier, faces new scrutiny into how he handled allegations of police brutality in his job as mayor of South Bend, Indiana.

Who's running?:Meet the candidates for president in 2020 in an interactive guide

When do we vote?:The full schedule for the 2020 presidential election primaries

Avoiding an 'oops' and looking strong

Other candidates, scrambling for traction, have been strategizing in practice sessions on how to deliver a viral moment of insight or humor that could prompt voters to take a serious look at them – and how to avoid the sort of gaffe that might undercut their prospects.

Exhibit A: Rick Perry.

Before the 2012 GOP primaries, Perry saw his brief front-runner status eroded by one weak debate performance, then demolished by a second. “Oops,” as the then-Texas governor memorably put it when he couldn’t remember the third federal agency he had proposed to abolish.

In the debates leading up to the 2016 Republican primaries, the contest that most closely resembles the big field and broad horizons of the Democratic race this time, a crisp showing by former Hewlett-Packard CEO Carly Fiorina boosted her fundraising and her status. For a time, it moved her from the undercard slate to the main event.

2020 debates:What the candidates will talk about on the national stage

Who's not here?:Bullock, Moulton and Messam didn't qualify for the debate

Primary debates get lower TV ratings than general election debates, when the landscape has been set between the parties' nominees, but they have more power to change voters’ choices within those parties. In a study at the University of Missouri, 40% of those who watched a primary debate supported the same candidate when it was over as they did when it began.

One in three switched from one candidate to another, and one in four went from being undecided to endorsing someone.

"The dynamic that's so different in a primary debate is that people's opinions aren't constrained by their partisanship," says communication professor Benjamin Warner, who conducted the study with Mitchell McKinney. "Everybody is a member of your team" and open for consideration. Though the impact isn't set in stone, it does show some resilience. "The initial impression they make in the first debate is a little bit resistant to change," he says.

In other words, a candidate will never have a second chance to make a first impression.

Not a lot of time for talking

They also won't have the luxury of time to make their case.

Answers are limited to 60 seconds and responses to 30 seconds. During each two-hour debate, featuring 10 candidates and a total of five moderators, the contenders can count on getting only six minutes or so to talk. Some will get more time to respond if they are the target of attack.

Just how many attacks are launched, and with what ferocity, is one key thing to watch.

Debate matchups:Biden vs. Sanders and other things to watch in the first debates

As the early front-runner, Biden, who is on the debate stage Thursday, is likely to be the prime target. There are risks to those who might take him on, especially among Democratic voters concerned about damaging the person who could end up as the party's standard-bearer. That means Biden's recent controversies, including his comments about his work as a young senator with segregationist colleagues, are more likely to be raised by the longest-shot contenders positioned at the edges of the stage.

Will they talk Trump or policy?

Another key: How big a presence is Trump on stage?

Biden is likely to try to pivot from an attack by a Democratic competitor to a conversation about the imperative to defeat Trump's bid for a second term. Since Biden announced his candidacy, he has argued he is the Democrat best able to compete with the president in such battleground states as Pennsylvania and Michigan. Trump may be bolstering the idea that Biden is the likely nominee by aiming most of his own fire at the former vice president, questioning why former President Barack Obama hasn't endorsed him.

Opposition to Trump and his agenda unites the Democratic field, to be sure. But the other 19 Democrats on the debate stage aren't ready to move on to the general election until the battle over the Democratic nomination has been waged.

USA TODAY/Suffolk Poll:What do Democrats want to hear about? Hint: Not Trump.

One more thing to watch: How fierce is the Democrats' policy divide?

The Democratic Party doesn't face the bitter fractures of the past, over the civil rights movement or the Vietnam War, for instance. But the candidates do divide on some major proposals, including the so-called Green New Deal to combat climate change and "Medicare for All" to overhaul the health care system.

Half the candidates – including Sens. Cory Booker of New Jersey, Kirsten Gillibrand of New York and Kamala Harris of California – have endorsed the idea of some form of Medicare for All, although not always with precise explanations of what that would include. Others, including Sen. Amy Klobuchar of Minnesota and Rep. John Delaney of Maryland, have raised questions about the impact on American workers covered by private insurance. They could speak up.

And while Sanders proudly declares himself to be a democratic socialist, former Colorado Gov. John Hickenlooper, a former business entrepreneur and capitalist, has argued that socialist policies and the label itself put Democrats at risk among swing voters.

He could speak up, too. Quickly.

'Off and running':A look at President Trump's campaign for reelection in 2020

Expanding the map:Why candidates are already campaigning beyond the first primary states