Most Democrats want to see Sanders treated with respect and dignity, said Larry Sabato, director of the University of Virginia’s Center for Politics. But Sabato also said the nomination battle is over and Sanders "has to cooperate.” Tuesday's meeting with his Senate colleagues offers Sanders a good chance to transition to the role of “party unifier,” Sabato said.

“If he drags his feet, he will be seen as a spoilsport at a time when Democrats are horrified at the prospect of a President Trump,” Sabato wrote in an email. “Sanders will risk losing all that he has gained.”

On Sunday evening, after meeting in Burlington, Vt., with his close advisers and supporters, Sanders called for a 50-state strategy to get young people and working people to run for political office and to demand that government answers the needs of “ordinary people,” not just wealthy campaign contributors.

Asked whether he'll still consider himself a candidate after Tuesday, Sanders responded: "We are taking this campaign and our ideas for a strong platform to transform the Democratic Party away from a party which spends far too much time raising money from wealthy people into a party which represents the grassroots of this country."