That resilience has created a challenge for many of the former vice president’s rivals as the summer comes to a close. Their routes to the nomination depend on winning over current Biden supporters, but his staying power has yet to offer a lasting opportunity to chip away.

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In response, top advisers to many of his rivals have counseled that the only path forward they see is to continue to cast their candidates as younger, more transformative or more energetic change agents, figure out how to maintain their spot on the debate stage, and hope that the mercurial history of Iowa and New Hampshire voters repeats itself, torpedoing Biden’s bid as they have not.

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“Come January, voters are going to give candidates a hard final look and make a decision about who to support,” said Justin Buoen, the campaign manager for Sen. Amy Klobuchar (D-Minn.). “Between now and then is a lifetime.”

But just what could bring down Biden remains an open question among campaigns. Some hope that his verbal struggles in debates and at campaign events, including recent misstatements in a tale of military heroism, could feed growing concern about nominating a man who would be the oldest U.S. president in history. Many discount the value of current polls, since so many voters have yet to focus on the choice and many are open to changing their minds.

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Others have grown increasingly skeptical that direct challenges to his record will change the dynamics of the race. Rather than risk alienating his supporters, they are choosing to build a broader argument about generational or policy change — an implicit contrast to Biden and his central pledge to “restore the soul” of the nation and revisit the popular parts of the Obama administration.

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“We can’t look like our message is to just, kind of, turn back the clock and go back to normal,” South Bend, Ind., Mayor Pete Buttigieg said on a campaign swing through Iowa last month. “There’s no ‘again.’ It’s about making sure that the future is better than the past and representing something that’s going to be new and different.”

Some candidates, including former housing and urban development secretary Julián Castro, have begun to more explicitly urge Democrats to move out of the safety of the familiar.

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“If you take a look at the modern era of presidential campaigns, when Democrats have won, it’s because they’ve taken a bit of a risk, whether it was Kennedy in 1960, or Carter in 1976, or Barack Obama in 2008,” Castro said Sunday on NBC’s “Meet The Press.”

This approach will again be on display at the next debate in Houston on Sept. 12, when Biden will for the first time be confronted by all of his opponents registering in the polls, each of them eager to contrast their visions and styles. For the first time, Sen. Elizabeth Warren (Mass.) will share the center of the stage with Biden after appearing on separate nights over the summer debates and mostly avoiding direct confrontation with him despite a long history of policy disagreement.

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It remains unclear, however, whether his rivals will repeat the direct assaults on Biden of the first two contests. At several points since he entered the race in April, such efforts against Biden have rewarded his attackers with attention, fundraising bumps and fleeting polling shifts. But they have also sometimes backfired.

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In the first Democratic debate in June, Rep. Eric Swalwell (Calif.) staked his campaign on demanding that Biden “pass the torch” to younger candidates, only to abandon his presidential effort days later. In the last big moment of her campaign, Sen. Kirsten Gillibrand (N.Y.) launched an assault on Biden at the July debate, arguing that a 1981 opinion piece he had written showed disrespect for women who work outside the home. She dropped out last week after failing to qualify for the September debate.

The most successful challenge came from Sen. Kamala D. Harris (Calif.), who denounced Biden’s opposition to school busing with a well-delivered personal story of her own childhood at a recently desegregated school. But she was unable to turn the moment into anything more than a temporary bump in fundraising and polling support.

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Her advisers remain convinced that the confrontation will help her campaign down the road, though since then she has not made Biden a major part of her message.

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“In the first debate, she showed strength and viability,” said Ian Sams, her national press secretary. “And we think that will stick in people’s minds for the rest of the primary.”

Part of the challenge for Biden’s detractors is the historically low appetite among Democrats for intraparty fights.

“Anytime you are in a multi-candidate primary, the aggressor is not the direct beneficiary of an attack,” said Jeff Link, an Iowa Democratic strategist who has worked for the presidential campaigns of Bill Clinton, Al Gore and Obama. “Typically someone else benefits because nobody likes an attack.”

That happened in 2004, when Rep. Dick Gephardt (Mo.) unleashed a blistering attack on Vermont Gov. Howard Dean days before the Iowa caucuses, calling him a “weather-vane Democrat” with “false conviction.” The attack helped sink polling support for Gephardt and Dean, allowing Sen. John F. Kerry (Mass.) to win the state.

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Biden, who has run for president twice before, knows this history well and has parried the attacks on him by admitting mistakes over the course of the campaign, including his decision to discuss his past working relationship with segregationist Democratic senators.

Before joining the race, he vowed without apologizing to be “more mindful and respectful of people’s personal space” after several women said his close physical contact had made them uncomfortable. Later, under direct criticism from many in the field, he reversed his position on federal funding for abortion, embracing it for the first time.

He has also warned his rivals that a fiercely negative fight could hurt the party, bringing the discussion back to his central contention that he is best able to defeat President Trump in a general election.

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“We shouldn’t be forming a circular firing squad and shooting, because all it does is help Trump,” he said during a meeting last week with black reporters in Washington.

His aides assert Biden’s longtime relationship with Democratic voters has allowed him to survive the attacks. Voters, they say, shrug off his gaffes and new facts about his history because they have watched him for so long in public life.

“When the press or other candidates make a big deal about something he says, they look at it and they roll their eyes,” said John Anzalone, a pollster for the Biden campaign. “Voters know Joe Biden. They have a feel for this guy.”

They also have firmly cast him as the candidate best positioned to defeat Trump, a high priority for most Democratic voters.

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“Democratic primary voters just want someone who is going to win, period, and right now he still occupies that space like no one else does,” said Glen Caplin, who served as a senior adviser to Gillibrand’s campaign. “But it’s still early, with a lot of time for that to change.”

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Biden’s two closest rivals, Warren and Sen. Bernie Sanders (I-Vt.), have so far taken different approaches toward him. Sanders has repeatedly tried to use Biden as an establishment foil for his anti-establishment campaign, holding him up as an obstacle to progress.

He accused Biden of “sounding like Donald Trump” in July on the subject of health-care policy, and has repeatedly called out Biden’s past support for free-trade deals and the Iraq War and criticized his approach to climate change as insufficient. At one point this summer, Sanders’s campaign released an online quiz that challenged voters to guess whether quotes had been uttered by Biden or Republicans such as Trump or Senate Majority Leader Mitch McConnell of Kentucky.

Warren, by contrast, has taken a far less confrontational approach, not just to Biden but to other Democrats in the race. Like Buttigieg and Castro, who also have focused on generational change, Warren has tried to build a broader thematic contrast with Biden, usually without mentioning his name.

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“Some Democrats in Washington believe the only way forward is to make change incrementally, a few tweaks here and some nudges there,” she told Democratic activists at a party meeting in San Francisco on Aug. 22. “No. This is a time of crisis, and when you’re in a crisis — when your back is up against the wall — the last thing you do is back down from the big fights.”

Through the first two sets of debates this summer, Warren and Biden did not appear on the same stage, but that will change in Houston. The moment could burst open an often tense private relationship between the two politicians.

Asked repeatedly in April about her past fights with Biden over a bill overhauling bankruptcy law, which she opposed and he supported, she was hesitant to go into detail about the split but nonetheless previewed the battle to come.