And this year, Warren could be rising in power just as a leadership vacuum is opening among Senate Democrats—and particularly those progressive populists. Reid is retiring, and his expected successor, Chuck Schumer of New York, is considered suspect due to his friendly relationship with Wall Street. Also retiring are two of the leading women of the Democratic Caucus: Barbara Mikulski of Maryland and Barbara Boxer of California. Warren’s only real rival in terms of national following would be Sanders, whose historic presidential run has built a loyal following outside of Washington. But, as Barney Frank has made abundantly clear in recent public tirades, Sanders’ progressive Capitol colleagues have always looked at him as a party of one, not a leader or even a team player. He became a Democrat only last year, and has publicly conceded that he did so only for self-interest; he continues to list himself as an Independent for Senate business. And Sanders isn’t doing himself any favors with them by criticizing what he calls a “rigged” nominating system of superdelegates—which includes all Democratic senators. His failure to help raise money for down-ballot Democrats hasn’t helped either.

Warren’s most powerful appeal is simply this: She’s the voice of the party. […]