Washington CNN  — 

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.

“She’s waiting way too long,” said one Democratic senator who asked not to be named.

READ: Clinton attacks as polls show Sanders winning in Iowa, New Hampshire

The potential endorsement also puts Warren in a bind. If she jumps behind Clinton, she will undoubtedly anger the legions of like-minded progressives who are powering Sanders’ insurgent campaign. But a Sanders endorsement will turn off not only women’s groups that back Clinton but also many of her fellow senators.

McCaskill noted not a single Democratic senator has backed Sanders, the Vermont independent who caucuses with Democrats.

“Probably all of us respect and love him, but we have a lot people here who have worked with him for many years – and he has yet to have one endorsement from people who know him well and who have worked with him,” McCaskill said. “There is a collective judgment there that one candidate is going to be better president, and I think that counts for something since we work with him every day.”

Warren has said previously she expects to give an endorsement, but it’s unclear when it will come. A Warren spokeswoman declined to comment.

Indeed, Democratic senators have lined up behind Warren - including the 13 other women in the Senate Democratic Caucus. In late November, Warren was the only Democratic female senator who skipped a fundraiser held at the Hyatt Regency on Capitol Hill in honor of Clinton. While she signed a letter in 2013 with fellow women Democratic senators urging Clinton to run, she has since said that did not constitute an endorsement.

READ: Obama hopes to pave way for Clinton with farewell State of the Union

“Of course she would love to have Elizabeth’s support,” Sen. Dianne Feinstein, D-California, said of Clinton. “It’s hard for me to get in anybody else’s mind as to why they would not.”

Privately, Warren met with Clinton at her Washington home in December 2014, and has since chatted with Sanders. Tad Devine, the senior Sanders strategist, said that Warren’s support would be “more than welcome” but he didn’t know what she would do.

“There’s no secret Elizabeth Warren plan,” Devine said. “She’s going to do what she wants to do when she wants to do it. That’s the way it goes.”

The chatter comes at an urgent time for the Clinton campaign. A Monmouth poll Tuesday found Clinton losing to Sanders by 14 points in New Hampshire – while a new Quinnipiac poll reported that Clinton was trailing by five points in Iowa.

Some Democrats are nervous about the prospect that Sanders could win the nomination.

“If Bernie was the nominee, I think creates a challenge,” said Sen. Joe Manchin, the moderate West Virginia Democrat. “But of course, Republicans have tremendous challenges, too.”

McCaskill said it’s time for Clinton to more sharply show the contrast between the two of them.

READ: Sanders: Clinton ‘in serious trouble’

“That’s what happens when someone remains undefined, and the other is so familiar to voters,” McCaskill said. ‘You’ve got a shiny new object, and you’ve got Hillary Clinton. That is a contrast in terms of familiarity. It’s not unusual for someone to go with the new and unfamiliar over something that is very familiar.”

McCaskill added: “I think the contrast is going to have to be drawn so people understand that Hillary Clinton is the only person running for president that is really qualified to lead in a stable and pragmatic fashion on the issues facing our country.”

Sanders repeatedly discounts the lack of support from his colleagues – even like-minded colleagues like Sen. Sherrod Brown, whom he referred to as “the establishment” after the Ohio Democrat endorsed Clinton.

“Bernie says, the next day, ‘I didn’t expect any help from the establishment,” Brown said. “I said to him, ‘Even though that kind of pissed me off, that was actually pretty good line.’ He smiled.”