The Working Families Party has endorsed Elizabeth Warren for the Democratic nomination and for president. This is something of a stunner, since the WFP endorsed Bernie Sanders last time.

The endorsement was made via a poll of the membership and leadership, in which Warren received 61 percent to Sanders’s 36 percent. It came several days after Warren astutely endorsed two high-priority candidates backed by the Working Families Party, one running for the Philadelphia City Council, the other being Stephen Smith, who is running a populist campaign for governor of West Virginia.

Warren also recently endorsed two progressive candidates backed by Justice Democrats, who are challenging center-right Democratic House incumbents, Henry Cuellar in Texas and Dan Lipinski in Chicago.

The Working Families Party is active in some 14 states, most notably in the Northeast. The fact that Warren could win handily among some of the most savvy progressive grassroots activists is another sign that she is gaining on Sanders as the champion of the party’s left.

“Senator Warren strikes fear into the hearts of the robber barons who rigged the system,” said Maurice Mitchell, national director of the Working Families Party, which pledged its organizing resources to help Warren become the nominee.

The endorsement and the astute politicking that led to it is a reminder that Warren is very deft at the inside game as well as the outside game. There was some grumbling, via tweets, from the Sanders camp that the Warren people and the WFP had played dirty.

This is a win for Warren—but also a warning that sooner or later, Sanders and Warren will inevitably have to shelve their tacit non-aggression pact, since only one of them can win.