Warren, Sanders appear to break nonaggression pact prior to debate

Elizabeth Warren claims that he told her in 2018 a woman couldn't win the presidential election and that his current campaign volunteers are trashing her to voters.
2:44 | 01/15/20

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Transcript for Warren, Sanders appear to break nonaggression pact prior to debate
And now to the race for 2020. The high stakes debate tonight. The war of words between Bernie Sanders and Elizabeth Warren. Did Sanders say a woman could not win the presidepresidency? Joe Biden to stand in the middle of them tonight, amid two new polls, one showing Bernie Sanders leading, one showing Biden leading. What does this all show? Very high stakes tonight. Linsey Davis from Des Moines. Reporter: Bernie Sanders arriving today at the debate hall in Des Moines. Elizabeth Warren checking out the podium. Iowans bracing for the clash of the liberal titans. I hope Bernie reconsiders and turns his campaign in a different direction. Reporter: The de facto nonaggression pact between the two Progressive senators seems to be out the door. Warren now charges that when she and Sanders met in 2018 to discuss the election, he told her a woman can't win. Among the topics that came up was what would happen if Democrats nominated a female candidate, Warren says. I thought a woman could win, he Sanders firing back. "It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president I would tell her a woman couldn't win. Do I believe a woman can win in 2020? Of course." Today, even one of their former rivals weighing in. Oh, a woman can be president. Of that I am sure. Reporter: Warren is also seizing on reports team Sanders is telling voters are chief supporters are "Highly-educated, more affluent people," not the blue collar voters needed to beat Donald Trump. I was disappointed to hear that Bernie is sending his volunteers out to trash me. Reporter: Sanders claims he had nothing to do with it. I have never said a negative word about Elizabeth Warren, who is a friend of mine. Reporter: With the Progressives tussling, the moderates in the field looking to rise above and break away from the pack, with those first crucial votes now just days away. All the polling data that I've read and seen is -- it's all, you know, it's a toss up. So, let's get to linsey Davis, live from Des Moines tonight. And linsey, the stakes even higher tonight to try to break through for these candidates, just about two and a half weeks now before the Iowa caucuses. Hard to believe. Because some of these candidates are going to be needed in Washington. They're going to be taken off the campaign trail for the senate impeachment trim. Reporter: That's right, David. Three of the six candidates, in fact, that are going to be on the stage tonight, they're going toeed to be back in D.C. For that impeachment trial six days a week. As a result, they're considering a number of options. Of course leaning on their surrogates, doing some town halls by tell conference and even potentially flying back and forth in order to make it back in time for trial the next morning. Crazy couple of weeks ahead. Linsey, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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