But he also opened two new lines of attack, knocking Mr. Biden for his past votes for the “don’t ask, don’t tell” policy that prohibited openly gay people from serving in the military and for his past support for the Hyde Amendment, a measure that bans federal funding for most abortions.

“When you’ve got two candidates, unlike when you have 18 candidates, it is possible to really contrast the views of the candidates,” Mr. Sanders said. “That’s what I’m going to do.”

There was a brief disruption at the rally in Phoenix when a man in the crowd unfurled a Nazi flag bearing a swastika, before he was quickly removed. “The senator is aware of the flag with the swastika on it and is disturbed by it,” said Mike Casca, a campaign spokesman. Mr. Sanders is Jewish.

A crucial part of Mr. Sanders’s argument has been the idea that he is the most electable candidate, able to defeat President Trump in a general election by appealing to the same working-class white voters who helped hand Mr. Trump his victory in 2016. But among aides and advisers, there has been a growing recognition that his claim hinges on his ability to demonstrate this strength in Midwestern states during the primary.

Mr. Sanders’s disappointing performance on Super Tuesday — he won only four states to Mr. Biden’s 10 — has only increased the sense of urgency inside his campaign.

At a news conference on Wednesday, Mr. Sanders said he was “disappointed” in the Super Tuesday results. And in an extraordinary concession, he admitted that his campaign had not managed to generate the soaring turnout among young people that he had banked on to secure the nomination.

While Mr. Sanders has managed to draw support in high numbers among other demographic groups, including Latino voters, his deficit with black voters in the South was central to his losses on Super Tuesday.