• Warren said she was "disappointed" in a Sanders volunteer script that implied she couldn't bring new voters into the party and confirmed reports that in a 2018 meeting Sanders told her a female presidential nominee couldn't win.
  • Insider SurveyMonkey Audience polling from the last two months shows Sanders and Warren share significant overlap, with 30% of Democratic primary voters being satisfied with both candidates.
  • In such a crowded primary field, Sanders and Warren will have to fight for every vote in order to lead the progressive lane.
  • Even if the two don't harbor any personal animosity towards each other, the numbers show why they'll need to fiercely compete on both policy and style to win over primary voters who are generally satisfied with both of them.
  • Visit Business Insider'shomepage for more stories.

With the leading Democratic presidential candidates are bunched in a close four-way race in both Iowa and New Hampshire, the previously cordial relations between the two progressive frontrunners Sens. Bernie Sanders and Elizabeth Warren are splintering and recent data shows why it was bound to happen.

For most of the 2020 campaign cycle, the two candidates have shied away from directly taking shots at each other after reportedly reaching an agreement in December 2018 not to attack each other during the campaign if they both ran.

But over the past few days, the previously friendly relations between the progressive senators have turned adversarial as the two and not just their surrogates have begun to duke it out in public, inflaming tensions that could spill onto the stage of the January 14 Democratic primary debate in Iowa.

To help make sense of where all these candidates stand, Insider has been conducting a recurring SurveyMonkey Audience national poll since December 2018. You can download every poll here, down to the individual respondent data . ( Read more about how the Insider 2020 Democratic primary tracker works .)

30% of Democrats like both Warren and Sanders. They'll have to choose.

Over the past seven polls conducted between early November and mid-January, Insider surveyed 2,963 Democratic primary voters on the state of the race, and who they would be satisfied with as the nominee.

Overall, 1,404 were satisfied with Warren as the nominee, and 1,400 were satisfied with Sanders. First up, it's worth noting that it is remarkably close. These are two candidates who are very appealing to a large and approximately equal fraction of the electorate.

The most significant figure is 903, which is the number of respondents who support both candidates. This comes out to 60% of their respective fan bases. Those 903 respondents are 30% of all the Democratic voters surveyed who would be satisfied with either candidate.

Granted, there's ample overlap with the other participants in the race as well, but concentrating on the Warren-Sanders faction is instructive because it illustrates how much is at stake here.

In Morning Consult's weekly national tracker of the Democratic primary, Warren is also the definitive second choice among Sanders' supporters, and vice-versa.

Bernie Sanders Dec 20
Bernie Sanders Dec 20

Over the weekend, Warren openly said she was "disappointed" in a Politico report detailing a Sanders volunteer script which the campaign tried to distance itself from emphasizing how Warren enjoys greater support among "affluent" and "highly educated voters" and claiming she isn't capable of bringing new voters into the party.

On Monday, CNN reported that Sanders told Warren in their 2018 meeting that he didn't believe a woman could win the presidency, which Sanders vehemently denied. CNN's reporting was later confirmed by The New York Times and BuzzFeed News , with the Warren campaign initially declining to comment on all three outlets' stories.

But after mounting pressure for her personally to speak out, Warren released a statement confirming the reports on Monday night. She said that in the 2018 meeting when they were discussing what would happen if the Democrats choose a female nominee, "I thought a woman could win, he disagreed."

While Warren's statement emphasized that she and Sanders share more common ground than disagreement, the campaign non-aggression pact the two reportedly negotiated over a year ago has now thoroughly broken down.

Even if the two don't harbor any personal animosity towards each other, the numbers show why they'll need to fiercely compete on both policy and style to win over primary voters who are generally satisfied with both of them.

Warren will benefit from Julian Castro's endorsement and Sens. Kamala Harris and Cory Booker dropping out of the race , and she is a popular second choice in key early states like Iowa, too.

Elizabeth Warren Dec 20
Elizabeth Warren Dec 20

While Sanders has been holding steady overall and placing first or second in recent surveys of Iowa and New Hampshire (albeit often within the margin of error), Warren has the unique advantage of being broadly liked by other candidates' supporters.

And in a state like Iowa where voters who pick candidates who don't initially clear the delegate threshold can redistribute their votes up to those who have met the 15% mark, being generally well-liked can make all the difference between receiving and not receiving delegates.

In a recent Monmouth University poll of Iowa , 23% of other candidates' supporters said they would caucus for Warren if their preferred candidate did not receive delegates, followed by 15% who would pick former Mayor Pete Buttigieg, 14% who would pick Sanders, and 10% who would choose former Vice President Joe Biden .

Walt Hickey contributed to this report.

Read more:

Here's who will be onstage for the tonight's Democratic presidential debate in Iowa, what time it starts, and how to watch it

Elizabeth Warren confirms Bernie Sanders told her a woman could not win the presidency

Sen. Cory Booker drops out of the 2020 presidential race less than a month before the Iowa caucuses

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