On Sunday, Ms. Warren said she was “disappointed” that Mr. Sanders’s campaign had been using a call script for volunteers that suggested she was appealing mainly to highly educated voters and would not be able to expand the Democratic Party coalition.

“I was disappointed to hear that Bernie is sending his volunteers out to trash me,” Ms. Warren said. “I hope Bernie reconsiders and turns his campaign in a different direction.”

After months of studiously avoiding any negative words about Mr. Sanders, Ms. Warren went on to cite the divisiveness of the 2016 primary between Mr. Sanders and Hillary Clinton, suggesting it 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.”