"Our campaign and her campaign had reached an agreement on a number of debates including one here in California in May," Sanders said during a rally in Santa Monica, California, Monday evening.
"I gotta tell you this. I think it is a little bit insulting to the people of California, our largest state, that she is not prepared to have a discussion with me about how she will help the Californians address the major crises we face."
Clinton issued a statement Monday declining an invitation to participate in the Democratic debate with the Vermont senator before the state's June 7 primary, instead looking ahead to the general election, the campaign said.
"We have declined Fox News' invitation to participate in a debate in California,” campaign communications director Jennifer Palmieri said in a statement. “As we have said previously, we plan to compete hard in the remaining primary states, particularly California, while turning our attention to the threat a Donald Trump presidency poses.
"We believe that Hillary Clinton's time is best spent campaigning and meeting directly with voters across California and preparing for a general election campaign that will ensure the White House remains in Democratic hands," she added.
Sanders also tweeted his "disappointment" in the former secretary of state's refusal to participate in the debate.
"I am disappointed but not surprised by Secretary Clinton’s unwillingness to debate before the largest and most important primary in the presidential nominating process," Sanders said. "The state of California and the United States face some enormous crises. Democracy, and respect for the voters of California, I would suggest that there should be a vigorous debate in which the voters may determine whose ideas they support.
“I hope Secretary Clinton reconsiders her unfortunate decision to back away from her commitment to debate.”
The self-proclaimed democratic socialist didn't let up. He added it's too soon for Clinton to claim victory, despite her daunting delegate lead.
"I also would suggest that Secretary Clinton may want to be not quite so presumptuous about thinking that she is a certain winner. In the last several weeks, the people of Indiana, West Virginia and Oregon have suggested otherwise,” he tweeted.
Bill Sammon, a Fox News VP and Washington managing editor, also issued a statement, expressing disappointment in Clinton's decision.
“Naturally, Fox News is disappointed that Secretary Clinton has declined our debate invitation, especially given that the race is still contested and she had previously agreed to a final debate before the California primary," he said in a statement.
The Clinton and Sanders campaigns had agreed earlier this year to extend the Democratic debate schedule by four more debates. Three have been held so far with the final debate supposed to take place in California in May -- before the state's June 7 primary -- according to the agreement. It's unclear now whether a fourth and final debate will be held at all.
Sanders last week accepted an invitation from Fox News to take part in a debate moderated by Bret Baier, Megyn Kelly and Chris Wallace. Fox sent letters to both campaigns formally inviting them to a debate.