“If they start going further down that negative road, I think we’re going to start looking at whether or not she can really tell the big banks to reform when so much of her money comes from Wall Street,” Mr. Devine said. “That would be Step 1. Step 2 would be: You go to Goldman Sachs and give a speech for an hour and get $200,000 — and you are going to be able to stand up to the banks?”

Mr. Devine said there was a line “between drawing sharp contrasts and going negative, and voters give you a lot of leeway when you are drawing contrasts on issues that they care about, like breaking up the big banks.”

“You’ll never see us going after her negatively, like with a TV ad that uses her face and systematically attacks and undermines her in a personal way,” he continued. “She can go as negative as she wants, but that’s just not who Bernie is.”

A spokeswoman for Mrs. Clinton said Mr. Sanders was the one going negative.

“Her campaign has been about how to make a difference in American families’ lives — a cause she’s fought for her entire life,” said the spokeswoman, Christina Reynolds. “It’s a shame Senator Sanders’s campaign has decided to make the campaign about political attacks. Voters want to hear about how their candidate will fight for them, not fight each other.”

Mr. Sanders’s own speech at the dinner struck some in the audience as too focused on comparing his positions and his campaign with Mrs. Clinton’s. “I think when an unconventional candidate starts using conventional strategies, there’s a dissonance to that message,” said Jeff Link, a veteran Democratic operative. “I think there’s a real risk to that.”

One Clinton supporter, former Gov. Howard Dean of Vermont, whose 2004 presidential run is often compared to Mr. Sanders’s, said that Mr. Sanders faced the difficulty of modifying his message to try to attract Clinton supporters and undecided Democrats instead of just pleasing his own liberal base.