Not long after meeting with Sanders at the end of 2018 to discuss her impending presidential run, Warren hosted an off-the-record dinner with a number of journalists, according to sources with knowledge of it. At the dinner, Warren was asked about her meeting with Sanders, and in the course of the discussion, she relayed that Sanders had warned that he didn't believe a woman could beat Trump in 2020. Different reporters recalled the comments differently, a mirror image of the dispute between Warren and Sanders over exactly what Sanders said — with Warren saying that Sanders argued a woman couldn't beat Trump, while Sanders said that he only said Trump would weaponize misogyny against a woman, not that it would work. (The Intercept was not at the dinner. Most politicians hold informal, off-record dinners or meetings with journalists, though it's not something Sanders is known to do. Occasionally details from those meetings leak, but it's rare.)
From there, the piece of news entered the journalistic bloodstream, circulating among reporters as gossip but not finding its way into print. On Monday, it finally did, with CNN's M.J. Lee reporting that according to four sources — described as "two people Warren spoke with directly soon after the encounter, and two people familiar with the meeting" — Sanders had told Warren, according to CNN's paraphrasing, that "he did not believe a woman could win."
It was widely assumed in the immediate aftermath of the story that Warren's campaign had planted the story. Indeed, CNN anchor Erin Burnett said as much on air. But Burnett was merely making an assumption, and had no inside knowledge of the sources, two CNN sources told The Intercept.
On Monday, Warren told The Intercept that her campaign did not intentionally plant the CNN story. That Warren told a number of journalists about the meeting a year ago adds context to that statement. If Warren had only told her closest advisers about the meeting, then it would be logical to assume that her campaign dictated the timing of the story, dropping it just ahead of a debate, and just weeks before the primary, to undercut Sanders. But since Warren told the story more broadly to a group of journalists, CNN's sources could have come from outside the campaign. The revelation does not rule out the possibility that someone in her campaign was a source, but it opens up other possibilities, as well.
In chat groups and in private conversations with people outside the campaign, Warren aides have insisted that they were not the source of the leak, and only learned about it in the midst of debate prep, contributing to the delayed response. The first time the campaign saw Sanders's on-record denial was in print in the CNN story. "What I did say that night was that Donald Trump is a sexist, a racist, and a liar who would weaponize whatever he could," Sanders said. After the story broke, the tension continued to escalate.
Oh and the Sanders camp looked into Warren being VP and Treasure Sec.