Voters under 45 continued to support Bernie Sanders by huge margins in Florida, Illinois and Arizona even as other groups came around to Biden. The gap has been largest with voters in their 20s or teens, mirroring a problem that hurt Hillary Clinton in key states: a lack of youth excitement.

“I’m deeply concerned about the impact that a lack of enthusiasm from young voters could have in a general election,” said Neil Sroka, a spokesman for Democracy For America, a progressive advocacy group that backs Sanders. “The consistent concern has been that nominating Vice President Biden would be essentially a repeat of the 2016 election.”

Failing to excite young voters in the primary has been a “significant red flag” for Democrats in recent decades, Sroka said: Barack Obama and Bill Clinton, who were backed by young people, went on to win the election, while Hillary Clinton, John Kerry and Al Gore lacked that enthusiasm and ended up losing.