Mr. Biden also had a tense exchange with a Sunrise organizer in September, who pressed him on his commitment to environmental issues.

“Look at my record, child,” Mr. Biden told the 18-year-old organizer.

The group’s announcement is one of the last presidential endorsements to arrive from major progressive groups, almost all of which have backed Mr. Sanders. His candidacy in 2016 helped develop much of the left-wing political infrastructure in the Democratic Party — and had huge support among young Democrats in particular — but when the 2020 campaign cycle began, it was not inevitable that he would retain that support.

Young Democratic voters had a bevy of options, including fresh faces like Mr. Buttigieg, former Representative Beto O’Rourke of Texas and Senator Kamala Harris of California. In Ms. Warren, there was another candidate with progressive bona fides — who would also be the first woman in the Oval Office if elected.

In September, the Working Families Party endorsed Ms. Warren over Mr. Sanders, saying that she was better positioned to create a cross-ideological coalition around liberal values and that grass-roots groups needed to choose a side in the primary.

Over the next several months, however, and after Ms. Warren drew significant criticism for a health care proposal that stepped away from an immediate push for “Medicare for all,” the energy on the Democrats’ left flank began to move away from her.

In October, Mr. Sanders announced endorsements from popular House Democrats including Ilhan Omar of Minnesota and Alexandria Ocasio-Cortez of New York, who is a sponsor of the Green New Deal legislation. He has also gained the backing of labor organizations such as National Nurses United, and left-wing advocacy groups including the Center for Popular Democracy Action and People’s Action.