Some members of the Sanders team became less fearful of a Warren run through 2018 as they watched the backlash to her DNA test and what they considered her underwhelming online fundraising. But as she’s rebounded the past several months, their mutual respect and solidarity in their broader progressive crusade is coming under duress.

The candidates themselves have largely kept things cordial. But signs of tension have trickled out. As Warren’s poll numbers ticked up, Sanders staffers took digs at her on Twitter for not participating in a Fox News town hall, while suggesting her rise was a manufactured media narrative. One anonymous adviser told U.S. News & World Report that her handling of the DNA test “just fundamentally killed her.”…

“Bernie and I have been friends for a long, long time,” said Warren in an interview, insisting their civil relationship will carry over to the debate stage. “I can’t imagine why it wouldn’t.” Asked by reporters last weekend in Iowa about what he expected from sharing a stage with Warren, Sanders replied: “Intelligence.”

Based on conversations with their advisers, it’s more likely the candidates will face off against the more moderate candidates onstage than against each other.