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."

Photos: Elizabeth Warren's career Photos: Elizabeth Warren's career Sen. Elizabeth Warren addresses a rally in support of Social Security and Medicare on Capitol Hill on September 18, 2014, in Washington, D.C. Hide Caption 1 of 17 Photos: Elizabeth Warren's career President Barack Obama (right) speaks as Warren and Labor Secretary Tom Perez listen at an AARP conference, February 23, 2015 in Washington, D.C. Hide Caption 2 of 17 Photos: Elizabeth Warren's career Warren (2nd from left) speaks as Senate Majority Whip Sen. Richard Durbin, D-Illinois, (left), Sen. Patty Murray, D-Washington, (3rd from left) and Sen. Chuck Schumer, D-New York, (right) listen during a news conference on college affordability on June 5, 2014, on Capitol Hill. Hide Caption 3 of 17 Photos: Elizabeth Warren's career Democratic Senators Barabara Mikulski, Debbie Stabenow, Warren, Tammy Baldwin and Amy Klobuchar at a news conference to announce their support for raising the minimum wage to $10.10 at the U.S. Capitol on January 30, 2014. Hide Caption 4 of 17 Photos: Elizabeth Warren's career Then-nominee for the Federal Reserve Board Chairman Janet Yellen (left) shakes hands with Warren (Right) after her confirmation hearing before Senate Banking, Housing and Urban Affairs Committee on November 14, 2013, on Capitol Hill. Hide Caption 5 of 17 Photos: Elizabeth Warren's career Warren speaks at a press conference on April 16, 2013, in Boston, one day after the Boston Marathon bombing. Hide Caption 6 of 17 Photos: Elizabeth Warren's career Senate Banking, Housing and Urban Affairs Committee members (right to left) Sen. Joe Manchin, Warren and Heidi Heitkamp participate in a hearing with then-Federal Reserve Bank Chairman, Ben Bernanke, after the release of The Semiannual Monetary Policy Report to the Congress on February 26, 2013, in Washington, D.C. Hide Caption 7 of 17 Photos: Elizabeth Warren's career (Left to right): John Kerry, Warren, then-Secretary of State Hillary Clinton and Sen. John McCain, are seated at Kerry's State Department confirmation hearings on Capitol Hill on January 24, 2013. Hide Caption 8 of 17 Photos: Elizabeth Warren's career Warren participates in a ceremonial swearing-in with her husband Bruce Mann and Vice President Joe Biden in the Old Senate Chamber on January 3, 2013, in Washington, D.C. Hide Caption 9 of 17 Photos: Elizabeth Warren's career Warren takes the stage for her acceptance speech after beating the incumbent Sen. Scott Brown on November 6, 2012, in Boston, Massachusetts. Hide Caption 10 of 17 Photos: Elizabeth Warren's career Then-Massachusetts Senatorial candidate Warren speaks at the Democratic National Convention on September 5, 2012, in Charlotte, North Carolina. Hide Caption 11 of 17 Photos: Elizabeth Warren's career President Obama (left) and former financial adviser Elizabeth Warren (left) arrive at the nomination of former Ohio Attorney General Richard Cordray (right) to lead a consumer protection bureau at the White House on July 18, 2011. Hide Caption 12 of 17 Photos: Elizabeth Warren's career Warren, the former Assistant to the President and Special Adviser to the Secretary of Treasury on the Consumer Financial Protection Bureau, testifies on TARP (The Troubled Asset Relief Program) and Bailouts of Public and Private Programs during a hearing on Capitol Hill on May 24, 2011. Hide Caption 13 of 17 Photos: Elizabeth Warren's career Warren (far right), an assistant to the President at the time, speaks at the Fortune Most Powerful Women summit on October 5, 2010, in Washington, D.C. Hide Caption 14 of 17 Photos: Elizabeth Warren's career President Obama (center), with then-Treasury Secretary Timothy Geithner (left) looking on, appoints former Congressional Oversight Panel Chair, Warren (right), to assistant and special adviser to the Secretary of the Treasury on the Consumer Financial Protection Bureau on September 17, 2010. Hide Caption 15 of 17 Photos: Elizabeth Warren's career (Left to right) Former Harvard Law Professor Warren talks with former White House Economic Recovery Advisory Board Chairman Paul Volcker and former Commerce Secretary Gary Locke before the signing ceremony for the financial reform bill on July 21, 2010. Hide Caption 16 of 17 Photos: Elizabeth Warren's career Then-Democratic presidential candidate Barack Obama (center) listens to questions with Rosa Figueroa (left) and Warren while hosting an economic round table at the Illinois Institute of Technology on June 11, 2008, in Chicago. Hide Caption 17 of 17

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.

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