As an intra-party debate over Wall Street reform has become a central question in the Democratic primary race, the eyes of America have turned to one woman, who has sat on a crucial endorsement for months and has the capital to continue withholding it: Massachusetts senator, Harvard Law professor, andanti-Big Bank crusader Elizabeth Warren.

Warren, who flirted with a presidential run for months (and the progressive world flirted back), could easily endorse either candidate without taking a reputational hit. By endorsing Clinton, she could cast herself as supporting the first female nominee of a major party and, possibly, the first female president. The rest of her female Democratic senators have already thrown in for Clinton, campaigning for her in their home states, and, according to The Hill, are nowurging Warren to do the same. (“I’ve told Sen. Warren that we would welcome her anytime she’s ready,” said Maryland senator Barbara Mikulski.) Given Sanders’s latest accusations that she is not progressive enough, Warren’s endorsement would serve as a critical bulwark for Clinton, who was recently forced to delay two finance-industry fund-raisers until after the New Hampshire primary.)

Politically, however, Warren is much more closely aligned with Sanders, a self-described democratic socialist and fellow New Englander whose campaign has taken up her crusade against big banks, money in politics, corporate corruption. In the past few days, Warren has released several statements implicitly supporting Sanders’s positions, delivering an impassioned speech on the Senate floor blasting “dark money” and defending him against Goldman Sachs C.E.O. Lloyd Blankfein’s assertion that a Sanders presidency would be “dangerous.”

“When Blankfein says that criticizing those who break the rules is dangerous to the economy, then he’s just repeating another variation of ‘too big to fail,’ ‘too big to jail,’ ‘too big even to prosecute,’” she said in an interview with International Business Times.

Unlike Bill de Blasio, who was in a similar position but quickly lost the favor of the Clinton campaign after the New York City mayor waited months to endorse her, Warren’s remarkable national popularity would be a major draw for supporters and donors on the campaign trail. She would also be an incredible vice-presidential pick for either candidate, bringing more female voters to Sanders or giving Clinton some much-needed progressive credibility. Most importantly, Warren can afford to wait for a clear winner to emerge, increasing her leverage over a future Sanders or Clinton administration—giving her another chance to run the Consumer Financial Protection Bureau, an agency she created but was denied the opportunity to lead—and, perhaps, bolstering her shot at a presidential run somewhere down the line.