SAN FRANCISCO — Sen. Bernie Sanders took aim at former Vice President Joe Biden on the final day of the California Democratic Convention Sunday, repeatedly declaring that there can be "no middle ground" when it comes to Democratic primary politics.

"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," the Vermont senator and socialist told the gathering in San Francisco to cheers from his supporters who swarmed the stage.

Liberal activists at the convention have taken a series of political shots at the more-moderate Biden for declining to attend the three-day event, while 14 other White House hopefuls addressed party faithful. Instead, the current front-runner of the crowded field jostling for the right to challenge President Trump next year elected to make remarks at the Human Rights Campaign's annual Columbus dinner in Ohio on Saturday night. Sanders ranks second behind the longtime senator for Delaware in polls.

"Let me be as clear as I can be. In my view, we will not defeat Donald Trump unless we bring excitement and energy into the campaign, 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," Sanders said. "We cannot go back to the old ways; we have to go forward with a new and progressive agenda. We have got to stand up and make it clear that we believe in a government, and an economy, and an energy system that represents all of us, not just the 1%.

"We have got to make it clear that when the future of the planet is at stake there is no middle ground," he added, repeating the phrase as he outlined his positions on healthcare, criminal justice, and defense spending.

Sanders' riff was an extension of earlier criticism he leveled at Biden after Reuters reported last month that Biden was set to propose a "middle ground" climate change policy in comparison to the more liberal "Green New Deal."

“If we don't commit to fully transforming our energy system away from fossil fuels, we will doom future generations,” Sanders said at the time. “Fighting climate change must be our priority, whether fossil fuel billionaires like it or not."