Elizabeth Warren says she is “disappointed” to hear that Bernie Sanders’ campaign had reportedly provided talking points to its volunteers, urging them to portray her as a candidate of the elites in contrast to the Vermont senator.

The attacks mark a shift from a reported agreement between the two progressives not to attack one another during the run up to the Democratic primaries next month, an agreement that has seen them teaming up on debate stages to fire outward at the other Democrats in the field to defend their shared progressive policies.

The two candidates have often been viewed as occupying similar real estate in the Democratic field, with both making a point of attacking billionaires and refusing to hold the types of big dollar fundraisers seen among those on the left as a corrupting force in politics.

"I was disappointed to hear that Bernie is sending his volunteers out to trash me," Ms Warren said of her Senate colleague, who currently leads the field in polls of Iowa Democrats, who will have first say in the primary season that starts in weeks.

She suggested a fight between the two campaigns could fracture the Democratic primary in a similar way Mr Sanders encountered in his protracted fight with Hillary Clinton in 2016.

"Bernie knows me and has known me for a long time. He knows who I am, where I come from, what I have worked on and fought for and the coalition and grassroots movement we are trying to build,” she said. “Democrats want to win in 2020 – we all saw the impact of the factionalism in 2016 and we can't have a repeat of that."

The comments came after a report from Politico surfaced indicating Mr Sanders’ supporters and surrogates had been given a script to tell voters in Iowa who expressed support for Ms Warren that her overall support was limited to the rich and educated — a message seen as an attempt to undermine her electability in an election potentially marked by populist politics.

The report came just after a key poll was released in Iowa by the Des Moines Register and CNN, showing Mr Sanders leading the field, and Ms Warren in second place.

While not the first time the two have sought to highlight their differences in the crowded primary field — they disagree on how to pay for their Medicare for All proposals and the timing of the roll out for that programme, for instance — Mr Sanders denied having approved the attack over the weekend, even as his campaign advisers mounted full throated attacks on Joe Biden for his record on the Iraq War.

“I got to tell you, I think this is a little bit of a media blowup, that kind of wants conflict,” Mr Sanders told reporters during a rare question-and-answer session in Iowa over the weekend.

“Elizabeth Warren is a very good friend of mine. 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.”

When pressed further, Mr Sanders, whose campaign has provided few details about the reported script, suggested that the whole thing may have been the product of an employee gone rogue: “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?”

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The Politico report published on Saturday night appears to show a call script for Sanders volunteers, which encourages them to attack Ms Warren. The Sanders campaign has not disputed the authenticity of the script.

“I like Elizabeth Warren. [optional] In fact, she’s my second choice. But here’s my concern about her. The people who support her are highly educated, more affluent people who are going to show up and vote Democratic no matter what,” it says.