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.

Ultimately, progressive or not, no presidential candidate in the United States can win without a proper structural and political response to the Black Lives Matter movement. Sanders bombed his Netroots appearance, but there are two precedents that progressives who support him can emulate: First, the Radical Republicans, who from the 1850s to 1870s championed the needs of white and black progressives. Backed by members of the Republican Party, the group rejected gradualism and demanded immediate citizenship for the enslaved, equal protection under the law and economic reparations for them, in the form of 40 acres and a mule.

Second, the populist movement of 1890s effectively represents how most progressives feel about government today. Black and white farmers in the South and Midwest fought corporate greed, white supremacy and both the Democratic and Republican parties’ lack of attention to their concerns by forming the Populist Party. As Warren articulated at Netroots conference, the Populist Party too wanted the government to regulate banks, prevent corporate monopolies and otherwise assuage economic strife.

The two movements could have significantly changed the trajectory of American history. But they were thwarted by white racial violence in the form of domestic terrorism. White supremacist groups, ranging from Confederacy lovers to the Ku Klux Klan, routinely destroyed crops, houses and livestock and killed black people. As anti-lynching activist Ida B. Wells-Barnet noted in her 1895 pamphlet “A Red Record,” white assaults targeted black individuals and communities that provided opportunities for economic competition for white communities during and after slavery such as in Wilmington, North Carolina; Chicago; and Tulsa, Oklahoma.

White progressives, including Sanders, must understand and address the destructive nature of slavery and white supremacy and the intersectionality of race, class, gender and sexuality in order to chart a new era of inequality and end the exploitation of black bodies and lives. Otherwise, a claim to being progressive or liberal will serve simply as an academic or intellectual exercise.

Sanders has since expressed his support for the Black Lives Matter campaign in a series of comments posted on Twitter after the hashtag #BernieSoBlack, which criticized his stances, trended nationally and made headlines. Other candidates have taken notice. Democratic front-runner Hillary Clinton, who skipped the Netroots conference, has since put out a statement and a video explaining her support for the campaign and a policy proposal on police body cameras. That will not be enough, but it is a start.

During this presidential election cycle, we need a clear-eyed formulation of a progressive policy agenda for the 21st century. Among other things, this includes raising the minimum wage, debt-free college, paid sick leave for families, reining in corporate influence in politics and strengthening Social Security, criminal justice and Wall Street reforms.

Addressing the needs and demands of the Black Lives Matter movement and knowing that black people’s voices and participation in our politics matter would be the first step in correcting these long-held gaps in the practice of American democracy.

Originally published by Al Jazeera America