SAN FRANCISCO — Sen. Bernie Sanders, a Vermont independent and 2020 presidential candidate, took a veiled shot at former Vice President Joe Biden on Sunday at the California Democratic convention, hitting him for not attending the progressive gathering without mentioning him by name.

"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," Sanders said in his speech at the San Francisco convention in an apparent reference to Biden, who is leading the crowded Democratic primary field in early polling.

"We cannot go back to the old ways," added Sanders, who is coming in second in many of those polls. "We have to go forward with a new and progressive agenda."

Democrats "will not defeat [President] Donald Trump unless we bring excitement and energy into the campaign," Sanders said. The Vermont senator, who also sought the Democratic presidential nomination in 2016, added that there can be no "middle ground" on a score of issues, ranging from environmental policy to health care.

More than a dozen 2020 Democratic candidates addressed the convention this weekend, but Biden instead attended the Human Rights Campaign's LGBTQ civil rights gala in Columbus, Ohio.

Acting California Democratic Party chairwoman Alexandra Gallardo-Rooker said the vice president discussed his absence with her in an 11-minute phone call on Wednesday, expressing his regrets for being unable to attend and noting his commitment to speak at the Ohio event.