As he fights anew for the Democratic nomination, it is not lost on him or his allies that his success hinges, in no small part, on his ability to capture that enthusiasm again — for both the optics of his race and the actual votes.

But this time he is no longer an insurgent, nor is he the only anti-establishment candidate in the race — factors that helped boost his standing among young voters. With the race entering the crucial fall period, other candidates, including Ms. Warren and Andrew Yang, have begun siphoning off some of his support.

[Sign up for our politics newsletter hosted by Lisa Lerer and join the conversation around the 2020 presidential race.]

Even Mr. Sanders’s closest advisers acknowledge that he cannot take for granted a voting cohort they view as critical, even if it is traditionally unreliable in actually making it to the ballot box.

“Last time it was much more organic,” Faiz Shakir, Mr. Sanders’s campaign manager, said. “And this time it’s far more intentional.”

In interviews, many young voters still praised Mr. Sanders, sometimes breathlessly — citing his authenticity and conviction but also his calls for free college and universal health care and his proposal to cancel student debt. But many also expressed curiosity, if not exuberance, about other candidates