After months of studiously avoiding any negative words about Mr. Sanders, Ms. Warren went on to cite the divisiveness of the 2016 primary race between Mr. Sanders and Hillary Clinton, implying it had helped President Trump. “We all saw the impact of the factionalism in 2016, and we can’t have a repeat of that,” she warned. “Democrats need to unite our party and that means pulling in all parts of the Democratic coalition.”

In a rare question-and-answer session with reporters after his final event of a weekend Iowa swing, Mr. Sanders — in response to a question on whether he approved of his campaign’s criticism of Ms. Warren — denied responsibility for the script, saying he himself had never attacked Ms. Warren. And he blamed the news media for overstating the tension between the two campaigns. “I got to tell you, I think this is a little bit of a media blowup, that kind of wants conflict,” he said.

“Elizabeth Warren is a very good friend of mine,” Mr. Sanders, of Vermont, said. “We have worked together in the Senate for years. Elizabeth Warren and I will continue to work together, we will debate the issues.”

“No one is going to trash Elizabeth Warren,” he added.

The Sanders campaign did not provide any further information on the script. Pressed again on the topic, Mr. Sanders said: “We have hundreds of employees. Elizabeth Warren has hundreds of employees. And people sometimes say things that they shouldn’t. You have heard me give many speeches. Have I ever said one negative word about Elizabeth Warren?”

The exchange on Sunday — just two days after a major Iowa poll showed them in first and second place in the state — was a rare fraying of a de facto nonaggression pact the two senators have shared since the beginning of the primary race. On the debate stage, they have often formed a progressive tag team of sorts, defending their far-reaching policy proposals against criticism from moderates that they were unrealistic. This was perhaps never clearer than when Ms. Warren, answering a debate question, declared “I’m with Bernie” on the issue of “Medicare for all.”