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’s aides are preparing him to do something that he’s never done before in his decades of public service: walk onto a debate stage as the party’s front-runner for the presidential nomination.

Biden has run for president twice before, both times as a huge underdog. Thursday night will mark the first time he takes center stage with a massive target on his back and 19 candidates itching to take him on.

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In debate preparation sessions, Biden’s aides have focused on two potential vulnerabilities — his propensity for long-windedness, which can lead to gaffes, and how to handle the pack of contenders that will be looking to cut him down or elevate their own candidacies through a viral exchange with the front-runner.

Biden’s rivals, including 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.), who will be standing next to him on stage, are already sharpening their attacks ahead of the debate, which marks the first major inflection point in the race to the nomination.

“The sharks smell blood in the water,” said one Democratic strategist.

The former vice president will enter the debate with a healthy lead over the rest of the Democratic field, both nationally and in the early-voting states.

But public opinion surveys also indicate that Biden’s polling strength may be weaker than it appears, with more than three-quarters of likely primary voters saying they’re still open to changing their minds.

And a weeklong controversy over Biden’s remarks about his common ground with two segregationist senators decades ago has raised new questions about his durability as front-runner.

The debate will now test Biden like he’s never been tested before.

During his 1988 and 2008 presidential bids, Biden was an underdog who was overshadowed by the bigger names in the race.

In 1988, Biden was running to be one of the youngest presidents ever elected, but quickly fell behind former Rep. Dick Gephardt (Mo.) and former Massachusetts Gov. Michael Dukakis and ultimately pulled out of the race under the weight of a plagiarism scandal.

In 2008, Biden was an afterthought in a race that quickly took shape around former Sens. Barack Obama Barack Hussein ObamaObama warns of a 'decade of unfair, partisan gerrymandering' in call to look at down-ballot races Quinnipiac polls show Trump leading Biden in Texas, deadlocked race in Ohio Poll: Trump opens up 6-point lead over Biden in Iowa MORE (Ill.) and 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 (N.Y.).

On the debate stage in 2008, Biden’s rivals repeatedly turned to him to praise his policy ideas or to cast him as a touchstone of liberal pragmatism.

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The praise was so effusive for Biden that his campaign cut an ad called “Joe is Right” that spliced together all of the instances in which Obama, Clinton and the other contenders praised his candidacy.

“I want to say amen to Joe Biden because he’s 100 percent right,” Clinton says at one point in the ad.

There will be none of that this time around.

“In his career, Biden has never been on stage in this context. He’s always been the underdog,” said Andrew Feldman, a Democratic strategist. “He’s walking into a completely different dynamic this time. No one’s going to be saying ‘Joe is right.’ They’re going to be coming at him from every which way.”

On Thursday night, Biden will be standing on stage with Sanders, who signaled over the weekend that he will go after the former vice president for voting to authorize war in Iraq under former President George W. Bush. Sanders is also preparing to cast Biden as a shill for corporate interests.

“Joe has to defend his record,” Sanders said on CBS’s “Face the Nation” on Sunday.

“I helped lead the opposition to the war in Iraq, which in my view was the worst foreign policy blunder in the modern history of this country. Joe voted for that. I led the opposition against disastrous trade agreements which cost the workers of this country millions of good-paying jobs. Joe voted for those. I voted against the deregulation of Wall Street, which in my view led to the great economic recession of 2008. Joe voted for them,” Sanders said.

Biden, 76, will also be on stage with 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, 37, who will be making the case that he’s a generational candidate and the party’s future. And there will be six low-polling candidates on stage, each looking to create a soundbite that rolls endlessly on cable news.

Meanwhile, the controversy over Biden’s remarks about finding common ground with segregationists in the Senate is certain to come up, although Biden’s fiercest critic about the comments, 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.), won’t share the debate stage with him.

None of the candidates broached the subject at a candidate forum in South Carolina over the weekend, but Biden came under attack in separate interviews on the Sunday shows by Sanders, Booker and 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.), who is African American and will be on the stage with him on Thursday night.

“We cannot be ignorant of the history of race in this country and certainly anyone who is a leader should not be,” Harris said on “Face the Nation.”

Biden’s aides and allies will also be on edge over his propensity to wander rhetorically, which can get him into trouble.

At a 2008 presidential primary debate, Biden was quick on his feet when moderator Brian Williams asked him if he could rein in his loquaciousness.

“Yes,” he said.

The one-word answer won over the room, but Biden’s inclination for verboseness will keep his advisers on edge until the night wraps up.

Others who have known Biden for a long time say it takes him a while to get warmed up and the first words out of his mouth are never his smoothest.

Allies wonder how Biden will do in a format where candidates will be limited to one-minute answers and 30-second follow-ups.

“He’s not great at keeping things tight,” said one Democratic fundraiser.

Still, as front-runner, Biden will have some advantages over the field.

Democrats say Biden won’t have to worry about attacking the other Democrats and can instead focus on drawing contrasts between himself and 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.

Tensions with Iran also give Biden a prime opportunity to cast himself as a steady commander in chief, and he’ll be able to point to polling that shows he is in the strongest position to defeat the president in 2020.

“Biden is a very skilled debater,” Feldman said. “But he’s never been in a position like this, and it will be fascinating to see how he handles it.”