Mr. Sanders, perhaps more than any other candidate, is betting his campaign’s success on his grass-roots appeal. He hopes to have an army of volunteers spreading his message on the ground — knocking on doors, handing out leaflets, engaging on social media — to say nothing of the money his campaign hopes to continue raising through individual donations. He is also aiming to get volunteers involved in a “massive voter-registration drive,” focusing in particular on working-class and young people, he said.

There have been signs his strategy is working: In the first quarter, Mr. Sanders’s campaign raised more than $18 million, outpacing the other campaigns by far. On the campaign trail, he often boasts that his campaign has signed up more than one million supporters.

But as his campaign prepares to compete with Mr. Biden’s, those aims have taken on new urgency. On Friday, Mr. Biden’s campaign announced it had raised $6.3 million during its first 24 hours, besting Mr. Sanders, who raised $5.9 million in his campaign’s debut. (Mr. Sanders, however, raised his first-day total from some 225,000 donors, more than double the roughly 97,000 donors who gave to Mr. Biden’s campaign, which held a big fund-raiser on Thursday night.)

Mr. Biden has also planned a campaign stop next week in Pittsburgh — union country — underscoring that he is homing in on the same blue-collar voters who were drawn to Mr. Sanders in 2016.

In the interview, Mr. Sanders said he was aware he was unlikely to draw the same level of support — from working-class people and others — that he enjoyed in 2016, given there are so many other candidates, including Mr. Biden, in the race.

“It is absolutely true that not only Joe Biden, but every other candidate, is going to be going after the support that we have had in the past,” he said. But, he said, he still thought his campaign was well-positioned for the battle ahead.

“I think we are doing well in hanging on to the support of the folks that we had last time — not 100 percent, that’s for sure — and also reaching out and bringing new people in,” he said. “But you know? That’s what the fight is about.”