“It’s like my going back and pointing out how Bernie voted against the Brady Bill five times while I was trying to get it passed when he was in the House, or how he voted to, you know, protect gun manufacturers,” Mr. Biden said. “It’s the only group in America you can’t sue. I mean, he’s made up for that. He’s indicated that was past.”

Mr. Biden was referring to Mr. Sanders’s opposition in the 1990s to a bill that required background checks for gun purchases, as well as his 2005 vote for a bill that shielded gun manufacturers and dealers from liability lawsuits.

As a group of reporters congregated around Mr. Biden at a campaign event in Mason City, Iowa, on Wednesday, Ed O’Keefe of CBS News pressed Mr. Biden on why his campaign had continued to criticize Mr. Sanders, even after Mr. Sanders had apologized for his surrogate’s op-ed. “Why wasn’t his apology enough?” Mr. O’Keefe asked.

Mr. Biden responded sharply.

“Why why why why!” Mr. Biden repeated, briefly grabbing Mr. O’Keefe’s lapels. “You’re getting nervous, man! Calm down. It’s O.K. He apologized for saying that I was corrupt. He didn’t say anything about whether or not I was telling the truth about Social Security.”

Mr. Sanders’s newly combative posture has been met with some relief inside his campaign: With the two men competing for an overlapping slice of working-class voters, some top aides have been quietly urging Mr. Sanders to draw more explicit contrasts with the former vice president. Not only would such an offensive help Mr. Sanders whittle away at Mr. Biden’s support, some advisers believe, but it would also satisfy supporters and donors to Mr. Sanders who crave a fight. Some also think he pulled punches against Hillary Clinton in 2016, to his detriment.

Since the fall, they have encouraged him to go after Mr. Biden aggressively on the debate stage, a strategy Mr. Sanders followed tepidly. During the debate last week in Des Moines, some advisers had prodded Mr. Sanders to confront Mr. Biden on Social Security and were frustrated the topic did not come up.

Mr. Sanders seemed to telegraph his willingness to engage in rougher campaigning during a question-and-answer session with reporters this month in Iowa City.