At the convention, Democrats made a major push to convert diehard Bernie Sanders supporters—especially his legions of devoted Millennials—into Hillary Clinton enthusiasts. Progress may have been made, but it’ll remain an ongoing and difficult task. If Clinton (or her rival Donald Trump) want to win their votes, they need to understand what these young voters found so attractive about Sanders.

Policy issues played a small role: His push for student loan forgiveness and making college and medical care free for users spoke specifically to the concerns of Millennials who have been struggling to make ends meet in this lackluster economy. But his rock star appeal stemmed from much more than just a liberal and pro-young policy agenda.

Much more importantly, Sanders spoke to Millennials’ aspirations and desire to stand out among their peers. His focus on making the world a better place, removing money from politics, creating an equitable society and particularly helping the worst off resonated with young idealists who want to change the world. The very fact that Sander’s campaign started as such a long shot was an asset with young voters: This gave them the opportunity to make a real difference. Bernie’s was the Kickstarter presidential campaign. He needed them to make it happen. As a result, Bernie Sanders wasn’t just succeeding as his campaign grew in momentum – his supporters were succeeding too.

Hillary’s slick campaign apparatus and overwhelming support from Democrat kingmakers and the party machine didn’t just remind Millennials that she is a Washington insider and would perpetuate business-as-usual, back-scratching politics. It also meant they were unnecessary. Hillary wasn’t going to depend on college-age volunteers to drive her campaign. Sure, they could vote for her, but they could never be the foundation of her campaign’s success.

Sanders also encouraged his followers to identify with his campaign and his candidacy. This wasn’t just about him, it was about all of them who were together making a movement. They were all “Berners,” a team with its own look, shirts, jewelry and other paraphernalia – many of which his followers designed and sold themselves – all of which built a feeling of unity. His brand allowed followers to feel as if they were the good guys, fighting the good fight against impossible odds. Berners were proud to stand with their man, and they were proud to stand together. They believed that supporting him said something good not just about the candidate, but about themselves as individuals.

Team Berner not only gave Millennials a sense of belonging, but also enabled them to stand out as separate from – and superior to – their politically uninvolved or even conventionally Democratic peers. They were the ones making a difference and speaking truth to power. They were the ones changing the course of history.

So what happens to this sense of identity and passion now? Clinton’s campaign team seems aware that they need to inculcate the feeling of being a part of something important, something morally right and authentic, as well as to create that sense of belong to attract more supporters, particularly the Millennials who used to back Sanders. Her campaign hocks a wide variety of paraphernalia, inviting her supporters to make supporting Hillary a meaningful social statement and trying to build a sense of community.

Yet Clinton has a big challenge to overcome the baggage from the bruising primary that didn’t just defeat Millennials’ preferred candidate, but that, in the eyes of Sanders’ supporters, even denied him a fair fight. The DNC didn’t just favor Clinton over Sanders, the DNC was rigging the system against them, discounting their voices and votes. That’s something that isn’t easily forgiven.

Millennials’ sour experiences with Clinton opens the door for Donald Trump to try to win them over. His biggest challenge will be convincing them that he is an aspirational figure and that his is a team that they’ll be proud to belong to. Millennials are the most diverse voting group and are passionately committed to the idea of inclusiveness. They recoil from the possibility of being seen as bigoted or judgmental; in fact, no insult could be worse. Trump will have to show that his immigration policies and tough talk about women and various ethnic groups are something different than plain old bias. He needs to convince them that he shares their desire to create a welcoming, diverse, but cohesive society.  His focus is to ensure that everyone ? including Americans who are already here and struggling in the current economy – are treated fairly and have the opportunity for a better life.

Bernie’s supporters are an attractive prize for any candidates – but they won’t be easily wooed. Candidates on the right and left should consider carefully, and learn from, what this unlikely political star did to earn such loyalty.