Bernie Sanders said 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.
Mr Sanders expressed concern that the reports of Mrs Clinton's victory came the night before "the largest primary" - California - and was annoyed by the way it was done.
He said 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," he said.
"The night before the largest primary, biggest primary in the whole process, they make this announcement. So I was really disappointed in what The AP did."
The AP's delegate count is based on pledged delegates won in primaries and caucuses, as well as a survey of superdelegates. The AP only included superdelegates in its tally if they said they would unequivocally vote for Mrs Clinton or Mr Sanders at the convention.
According to the AP's count, Mrs Clinton secured the commitments of enough delegates to become the presumptive nominee on Monday night, the day before primary elections in California, New Jersey and four other states.
There was anger expressed by some Sanders supporters, who vehemently protested against the decision of the AP and several television networks to report on Mrs Clinton's delegate count on Monday night.
"When the mainstream media calls the polls, calls the election, because they're already planning to do it to suppress the vote in California, we will fight on," Nina Turner, a former Ohio state senator, said.