The Vermont senator also said some Democrats believe the voters who backed President Donald Trump are racists, xenophobes and "deplorables," a word Clinton famously used.
"I do not agree," he said.
"It wasn't that Donald Trump won the election, it was that the Democratic Party lost the election," Sanders added.
He ran through Democratic losses in recent years -- checking off the Republican-controlled House, Senate, governor's offices and state legislatures -- before saying that Republicans are "a right-wing extremist party who has an agenda that most Americans soundly and roundly disagree with."
"How in God's name do they win elections?" Sanders said. "And the reason is, in my view, that the time is long overdue for fundamental restructuring of the Democratic Party. We need a Democratic Party which is not the party of the liberal elite but a party of the working class of this country.
"We need a party which is a grassroots party, a party where candidates are talking to working people -- not spending their time raising money from the wealthy and the powerful."
Sanders' comments came at a rally organized by his Our Revolution group, as well as several local left-wing organizations. Warren was not beside Sanders when he took the shots at Clinton's campaign and Democrats, and the Massachusetts senator and fellow 2020 Democratic prospect did not address those remarks onstage.
But the event was meant to be much more of a progressive pep rally than a rehash of the 2016 campaign.
Both Sanders and Warren touted proposals to make college affordable and to increase the minimum wage to $15 an hour. And both repeatedly lambasted Trump's presidency.
Sanders also touted his "Medicare-for-all" health care proposal, which he said he will introduce as legislation within the next month.
"On every major point facing this country, the American people do not believe in a right-wing agenda; they believe in a progressive agenda," Sanders said. "We've got to take that agenda to the people."