Washington (CNN) Bernie Sanders on Sunday took a tacit jab at former Vice President Joe Biden, calling out his Democratic presidential opponent without mentioning him by name for not appearing at the California Democratic Convention -- where more than a dozen other 2020 candidates made their cases to scores of Democratic voters.

"As you all know, there is a debate among presidential candidates who have spoken to you here in this room and those who have chosen for whatever reason not to be in this room about the best way forward," the Vermont senator said as attendees booed. "So let me be as clear as I can be: in my view, we will not beat Donald Trump unless we bring excitement and energy into the campaign."

He continued, "And unless we give millions of working people and young people a reason to vote and a reason to believe that politics is relevant to their lives."

Although Sanders did not directly name Biden, the former vice president's absence was explicitly noted by the group Action for a Progressive Future, whose members handed out fliers at the event that read: "Where is Joe Biden?"

Alexandra "Alex" Gallardo Rooker, acting chairwoman of the California Democratic Party, said at one point during the event, however, that she and Biden spoke last week and the former vice president said he had already committed to the Human Rights Campaign Dinner on Saturday night in Columbus, Ohio, and therefore could not make it to the convention.

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