On July 18, two candidates for the Democratic presidential nomination — former Maryland Gov. Martin O’Malley and Vermont Sen. Bernie Sanders — faced protests during a town hall discussion at the liberal Netroots Nation conference in Phoenix. The Black Lives Matter campaign, which is effectively merging online and offline activism against racial injustice, staged a demonstration in the conference hall, asking the candidates, Which side are you on?

Initially, O’Malley handled the protest well, even tapping his hands to the rhythm of a song the group sang and starting his remarks by answering questions about criminal justice and police reform. But he fumbled, inviting boos from the majority white progressive crowd when he said, “Black lives matter, and white lives matter, and all lives matter.” He should have known that the phrase “black lives matter” is part of an attempt to eradicate the racist and structural inequalities in the criminal justice system, which so often manifests as police brutality and other state-sanctioned violence. Hence, the emphasis upon black lives.

Sanders, whose appearance followed O’Malley’s, was given a rehearsal and a series of talking points by two Black Lives Matter activists who took the stage during the protest — Tia Oso and Patrisse Cullors. But clearly unmoved, he stepped off on the wrong foot by gesturing to the protesters and the audience to settle down so that he could get to “some serious things” he wanted to address, as if their questions were a mere frivolous disruption from the major issues facing the United States. He then proceeded to give his prepared campaign speech and threatened to leave if the audience “did not want him” to be there.

The incident, which followed a well-received keynote address by Massachusetts Sen. Elizabeth Warren a day earlier, laid bare a major political divide among white progressives and may now serve as the foundation for challenging other 2016 presidential contenders.

Unlike the two candidates, Warren did not simply recite the phrase “black lives matter”; she effortlessly connected the fight for racial justice with the struggle for economic justice in this country. “It shouldn’t take a revolution on YouTube to drive a revolution in law enforcement,” she said. “It shouldn’t take a hurricane in New Orleans or a massacre in Charleston for Americans to wake up to what is happening — what is still happening — to people of color in this country.”

Sanders maintains that the rise of “the billionaire class” is the key problem facing the United States today. And he believes that racial injustice is an outcome of economic injustices and will be solved only through an economic revolution. Racism is not simply a byproduct of class oppression and economic exploitation. Race and class are inextricably linked to the institution of slavery and, thus, the rise of capitalism in the United States, which dehumanized and objectified black lives by separating families and creating race-based, structural inequalities with clear economic outcomes.

In this regard, the Black Lives Matter protest was a teachable moment for everyone, particularly for white progressives. White progressives need to address racial injustice by picking a side. Will they be Warren progressives, who incorporate racial justice and economic justice? Will they be Sanders progressives, who prioritize economic justice as the cause of racial injustice? Or will progressives create an even better platform than what either of these politicians has articulated? Their answers are critical to shaping the conversation and electoral agenda in 2016.