Grijalva was Sanders’s first congressional endorser in 2015, while Abramson was a Sanders delegate to the Democratic convention and Sullivan was among his early backers in Iowa. Now all of them, and others in the same position, are signaling to their followers and allies that Warren is the better pick, quietly providing her an extensive network to build support in early states.

Among the guests at Abramson’s party were Mike and Jessica Smith, who backed Sanders last time. “She’s almost the new Bernie for us,” Jessica Smith said. “Bernie is a bit on the older side.” She added, “I still like Bernie. I still like his ideas, but she’s been able to add to that.”

This shift comes as Warren is publicly projecting a friendly attitude toward the Vermont senator — backing him up on the debate stage, refusing to criticize him when reporters ask, restraining her staffers from posting tweets needling him.

That avoids alienating Sanders voters whom she may need later. But strategists for both candidates say there’s only room for one of them to survive far beyond the early primaries, making for a below-the-surface battle, especially in Iowa and New Hampshire.