During Wednesday’s event, he made an explicit pitch to young voters, who largely supported Senator Bernie Sanders of Vermont in the Democratic primary. Youth-led climate groups like the Sunrise Movement endorsed Mr. Sanders, and they are among the constituencies Mr. Biden needs to win over for the general election.

“I remember, as you do, when President John F. Kennedy put out his inspiring challenge to put a person on the moon and return him safely in 10 years,” Mr. Gore said, referring to an address Kennedy gave to Congress in May 1961, when Mr. Gore was 13 and Mr. Biden was 18. “And I remember the adults of that day and time, many of them saying that’s a reckless, expensive, unwise venture.”

But when Apollo 11 landed on the moon eight years later, he continued, the average age of the system engineers in NASA’s mission control center was 26.

“That means their average age when they heard that speech was 18, and they changed their lives and they got the knowledge and the learning to be a part of that mission,” Mr. Gore said. “That’s what I feel from the young people in this country today where the climate crisis is concerned.”

It remains to be seen whether his endorsement will sway the young progressives who voted for Mr. Sanders. What it certainly does is reinforce the central image Mr. Biden’s campaign is trying to present: a Democratic Party that has moved on quickly from its hard-fought primary and united around its presumptive nominee in the face of a once-in-a-lifetime threat.

Within weeks after he essentially wrapped up the nomination, Mr. Biden has received endorsements from former President Barack Obama, Mr. Sanders and Senator Elizabeth Warren of Massachusetts, another primary opponent.

His campaign has unveiled several major endorsements in quick succession as something of a show of force.