Sanders to meet with delegates before start of convention

The planned closed-door meeting comes as many of his delegates are expressing disappointment and some uncertainty as they prepare to descend on Philadelphia for a weeklong convention in which Clinton will be formally nominated as the party's standard-bearer.

First lady Michelle Obama and Sanders were scheduled to address the convention that evening.

At the least, many want to see a traditional roll call vote of states, while others were considering mass sit-ins or even walkouts if delegates feel their views are not being respected or acknowledged, said Karen Bernal, a delegate from Sacramento, California, who helps lead the Bernie Delegates Network, which include more than 1,100 delegates.