Biden has been racking up endorsements of vanquished Democrats the past two weeks, including Pete Buttigieg, Amy Klobuchar, Kamala Harris and Cory Booker.

Yang backed Bernie Sanders in 2016 and frequently spoke about the senator as an inspiration for his own campaign. Yang's policies are considerably more liberal than Biden's.

Yang had wavered on whether he would be endorse any of his opponents if he dropped out. At one point, Yang told MSNBC, “If anyone wants my endorsement, all you have to do is come out for universal basic income. Say every American should get $1,000 a month, and then I will be there with you on the trail the next day.”

Yang said Biden asked for his endorsement as recently as last week when they spoke on the phone. “It was hard to say 'no' to Joe at the time,” Yang said on CNN. But if Yang made up his mind to back Biden during that conversation, he didn't share his decision until he went on air Tuesday, according to Zach Graumann, his former campaign manager.

Yang’s biggest base of support during his campaign was similar to Sanders': younger voters disillusioned with politics. He said needs to find a way to do appeal to them during the general election, assuming he wins the nomination.

“If Joe says, 'This is going to be a return to business as usual,’ he’s in danger of losing many of the young people that came out for Bernie, that supported my campaign," Yang said. "He has to show that, look, we know that this economy is not working for many, many Americans."