Sanders disappointed and upset at AP's declaration of Clinton's win

Democratic presidential candidate Sen. Bernie Sanders, I-Vt., and his wife Jane Sanders arrive at a campaign rally on Monday, June 6, 2016, in San Francisco.

LOS ANGELES - Bernie Sanders said Tuesday he was "really disappointed" and "upset" that The Associated Press declared rival Hillary Clinton the presumptive Democratic nominee on the eve of primary elections in six states, adding a somewhat muted voice to the outrage expressed by many of his supporters.

In an interview with NBC News, Sanders expressed concern that the news of Clinton's victory came the night before "the largest primary" and that it was based on what he described as "anonymous" commitments from superdelegates - the party insiders and Democratic officials who can vote for the candidate of their choice at the summer convention.

"They got on the phone as I understand it, and started hounding superdelegates to tell them in an anonymous way who they were voting for.