CEDAR RAPIDS, Iowa Tensions between the presidential campaigns of Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders , the two leading progressive candidates for the Democratic presidential nomination, are at an all-time high after the two candidates accounts of a 2018 meeting sharply diverged.

In a Monday night statement, Warren confirmed a CNN report that Sanders had told her in a late 2018 meeting that a woman could not win the presidency. I thought a woman could win; he disagreed, she said.

The Sanders campaign has steadfastly denied the Vermonter said anything of the sort.

It is a lie. Bernie Sanders has always stood for women and womens rights, Sanders campaign manager Faiz Shakir told CNN .

The fight between the two progressive New England senators was the second in as many days and seemed to effectively end a truce that had held since the earliest days of the presidential race. It followed a dustup over a Politico story showing a Sanders campaign script for volunteers casting Warren as the candidate of the affluent who would bring no new bases into the Democratic Party .

Both campaigns have largely declined to officially elaborate on the escalating disputes, which come three weeks ahead of the all-important Iowa caucuses and just one night ahead of a debate in Des Moines. A Des Moines Register/CNN poll of the caucuses, released Friday, showed Sanders leading the contest, with Warren, former Vice President Joe Biden and former South Bend, Indiana, Mayor Pete Buttigieg closely behind.

While the battle is relatively tame by the standard of past presidential primaries neither Warren nor Sanders has attacked the other with paid media or incorporated critiques into their stump speeches the possibility of a broader clash between them has alarmed many progressive organizations, who fear the fights only beneficiary would be the more moderate Biden.

Bidens lead in national polling and especially his strong standing with Black voters has made him the front-runner for the Democratic nomination.

I think theres a view among some people in the progressive community that Joe Biden will be very easy to beat, said Sean McElwee, the co-founder of Data For Progress and an occasional adviser to Warren. But Joe Biden is like the final boss in a video game, and the progressive candidates are rapidly eroding their health bar by fighting each other.

Democracy for America, another progressive group, has been scrambling to get the two campaigns to lower the temperature, with little apparent success so far.

There are only two candidates who are ready to fight for Medicare for All and the Green New Deal, and those are Elizabeth Warren and Bernie Sanders. And we need them working together, said Charles Chamberlain, the groups chair. Dont let the corporate wing or Donald Trump divide us.

Chamberlain suggested the two campaigns should work together to quickly end the dispute: Just like this thing can blow up out of nowhere, they can put it out very quickly, he said. We dont see Biden and Buttigieg fighting each other right now. They understand thats not in their best interest.