In an online appearance on Monday with Mr. Biden, Senator Sanders made a bid for Democratic solidarity and declared President Trump “the most dangerous president in the modern history of this country.”

Senator Sanders, who suspended his campaign last week, told Mr. Biden: “We need you in the White House. I will do all that I can to see that that happens, Joe.”

Mr. Biden vowed that the United States would “take a back seat to no one when it comes to fighting climate change.”

But winning over Senator Sanders’s most ardent supporters might still be a challenge for Mr. Biden.

Just a few hours before Senator Sanders’s announcement, for instance, I spoke with the leaders of The Climate Mobilization, an advocacy group that has worked closely with Mr. Sanders. The group’s director of research, Laura Berry, said Mr. Biden needed to offer more details.

“What the climate movement is looking for is for him to back up the rhetoric of saying things like ‘We need to get off fossil fuels’ with specific plans,” Ms. Berry said. “We don’t have any time left. We are in a climate emergency.”