Joe Biden’s fellow candidates have compared him to Donald Trump, dismissed him as past his sell-by date and gone public in unison with concerns about his previous positions on race, abortion and even his physically affectionate campaign style.

There is little evidence that any of it has stuck. Most August polls showed Biden with the support of nearly 1 in 3 Democratic voters nationally, far ahead of his nearest presidential opponent and basically unchanged from polling before he announced his campaign.

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.

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.

“Come January, voters are going to give candidates a hard final look and make a decision about who to support,” said Justin Buoen, 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.

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.

“We can’t look like our message is to just, kind of, turn back the clock and go back to normal,” said South Bend, Ind., Mayor Pete Buttigieg 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 Julián Castro, the former housing and urban development secretary, have begun to more explicitly urge Democrats to move out of the safety of the familiar.

“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 of Massachusetts 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.

It remains unclear, however, whether his rivals will repeat the direct assaults on Biden of the first two contests. Part of the challenge for Biden’s detractors is the historically low appetite among Democrats for interparty fights.

“Anytime you are in a multicandidate 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. Richard Gephardt of Missouri 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 Kerry of Massachusetts to win the state.