Sen. Elizabeth Warren is facing growing pressure from Senate Democrats to get behind Hillary Clinton as the former secretary of state suddenly finds herself struggling to keep pace with Bernie Sanders in early primary states.

In interviews with CNN, Democratic senators are grumbling over Warren’s refusal to pick a side, arguing that the populist liberal firebrand could help unite the party behind Clinton, whom they believe represents their only chance of winning the White House.

“I think it would be important,” Sen. Claire McCaskill, D-Missouri, said of Warren backing Clinton. “I think it would be helpful.”

At least one senator, Michigan Democrat Debbie Stabenow, has asked Warren to endorse Clinton, according to other Democrats. Stabenow’s office declined to comment.

Warren, whose influence with core Democratic voters is rivaled only by Sanders himself, could provide the Clinton campaign the boost it needs after two polls suddenly showed her trailing to Sanders in Iowa and New Hampshire.