MIAMI — Twenty Democrats have arrived here for two nights of heated debate over who is best suited to be the party’s presidential nominee and take on President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE in 2020.

Sen. Elizabeth Warren Elizabeth WarrenHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Democratic senators ask inspector general to investigate IRS use of location tracking service MORE (D-Mass.) will be the main attraction on Wednesday night, with former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE, Sen. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE (I-Vt.), Sen. Kamala Harris Kamala HarrisHundreds of lawyers from nation's oldest African American sorority join effort to fight voter suppression Biden picks up endorsement from progressive climate group 350 Action 3 reasons why Biden is misreading the politics of court packing MORE (D-Calif.) and South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The 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 MORE sharing the stage on Thursday night.

Here are five things to watch for:

A Biden pile-on is coming

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Biden has been the clear front-runner in the race since officially announcing his candidacy in April.

Now, the former vice president’s rivals will have a chance to cut him down in front of a national audience of millions.

“He can’t let his critics walk all over him,” said one longtime Biden ally. “He needs to go on offense and play defense at the same time.”

Can Biden stay above the fray? In recent debate sessions, Biden has focused on his own candidacy and why he’s the best choice to address the nation’s challenges and beat Trump. The famously loquacious Biden will have limited time to make that case, something that’s proven to be a challenge for him. Candidates will have only 60 seconds to respond to questions and 30 seconds for follow-ups.

Meanwhile, the former vice president has given his critics ample ammunition to come after him in recent weeks. Biden’s remarks about finding common ground with segregationist senators decades ago has caused a stir and is certain to be brought up, either by the moderators or the candidates themselves.

Sanders, who will be on stage with Biden, has signaled that he’ll make an issue out of Biden’s vote to authorize military action in Iraq, a criticism that he also leveled at presidential nominee Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE in 2016. And Sanders will look to cast Biden as beholden to corporate interests, highlighting his past support for the North American Free Trade Agreement and the credit card industry.

Harris, the former attorney general of California, has attacked Biden for supporting a 1994 crime bill that critics say led to the disproportionate incarceration of black people.

And 37-year-old Buttigieg will be on stage with Biden, 76, making the case that it’s time for new leadership and a generational change within the party.

An opportunity for Warren

The run-up to the first presidential debate has been a dream stretch for Warren.

The glowing media profiles are rolling in. Her poll numbers are up, and she’s caught, and in some cases, surpassed Sanders, who is her prime competition to be the party’s progressive standard-bearer. Warren’s flood of ambitious policy proposals has been a hit with liberals.

Warren will be the only candidate polling in double digits on the first night of the debate.

Her greatest competition for air time will be Sen. Cory Booker Cory Anthony Booker3 reasons why Biden is misreading the politics of court packing Bipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility MORE (N.J.) and former Rep. Beto O’Rourke (Texas), neither of whom is setting the primary race on fire.

Some believe that Warren could suffer by not being on stage with the other top contenders.

But if Warren can command the stage as she has in recent town halls and campaign events, she’ll be in good shape to continue her upward trend into the next phase of the race.

“Warren has one of the easier debate strategies because what she’s been doing on the campaign trail is already working, so she just needs to keep doing that,” said Democratic strategist Eddie Vale. “A lot of people will be tuning in for the first time, so she just needs to keep telling her great policy ideas interwoven with her bio and personal stories like she has been on the trail.”

Can Sanders recapture his old magic?

That Warren has caught Sanders in the polls has been one of the big surprises of the 2020 Democratic primary race so far.

Sanders’s backers say they’re not alarmed and blame the media for overplaying his decline. Indeed, some Democrats interviewed by The Hill insist that Sanders’s supporters will come home to him once voters begin casting ballots. He already has infrastructure in place from 2016 to capitalize on his existing base of support.

Either way, the stakes for Sanders at this first debate are monumentally high, as he seeks to cut into Biden’s support with Warren breathing down his neck.

Sanders proved himself a worthy debater in 2016, and his blunt talk about Clinton’s corporate speeches and close ties to the establishment propelled his unlikely candidacy and signaled the arrival of the progressive left on the national stage.

Now Sanders must make the case that he wasn’t merely a candidate suited for that time and place, but that he’s best positioned to take on Trump in the election.

Larry Cohen, the chairman of Our Revolution, a group that backs Sanders, said the Vermont senator is the strongest candidate for rural voters and unaffiliated voters “and he needs to make that clear” in the debate.

“The environment, opposing big money politics and ‘Medicare for All’ are all key for those voters,” Cohen said. “These voters will be the path to beating Trump.”

Trump looms over the Democratic field

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The president is thinking about live-tweeting the Democratic debate, but his specter will hang over the field in more ways than that.

Democratic voters say the most important quality in a candidate is their ability to defeat Trump.

Right now, Biden and Sanders poll the best against Trump in head-to-head match-ups, but the other candidates will be pressing their case as to why they should be the one given a chance to take the president down.

There are some fears among liberals that this field of candidates is too boring and won’t be able to match Trump’s energy. There have been criticisms that the contenders have allowed Trump to set the political agenda.

Voters will get an idea about what kind of star wattage these Democrats have when the TV ratings come in for the debates being hosted on NBC, MSNBC and Telemundo.

Is national enthusiasm around the field as high as it was in 2015, when 24 million people tuned in to the first GOP presidential debate? Or is the primary race at this point mostly of interest to Washington insiders and those who are obsessed with politics?

Long shots look for a moment

Of the 20 candidates who will debate over the two nights in Miami, 15 of them will be fighting for relevancy.

There is a clear top tier of Biden, Sanders, Warren, Harris and Buttigieg, although Buttigieg and Harris will be looking to reset their campaigns, having plateaued after high-flying starts.

The rest of the contenders are desperate for a standout moment that sets their candidacies on a new trajectory.

Having that many wild cards injects a level of volatility into the debate that could be a game changer for some low-polling candidate.

“A lot of these candidates have nothing to lose so they’ll be trying to land any punch, even if it’s a low blow,” said one Democratic strategist. “Anything for a cable TV moment.”