You might expect that Netroots Nation, the progressive conference whose 2015 edition was held in Phoenix last weekend, would be natural Sen. Bernie Sanders territory. And you could point to his speech on Saturday night, which turned out 11,000 people, as proof that you were right. But there's more than one kind of progressive.

Sanders's Netroots Nation appearance at a town hall Saturday afternoon turned into a confrontation with #BlackLivesMatter activists — and brought a conflict between Sanders-loving economic progressives on one side, and organizers for racial justice on the other, out into the open. But while Sanders is the catalyst, the conflict — at least as Sanders's critics see it — isn't really about whether to support Sanders or Hillary Clinton for the 2016 nomination. It's about who gets to call himself a progressive champion, and when politicians should heed activists' demands to pay more explicit attention to certain issues.

What happened at Netroots Nation

Two Democratic candidates appeared at Netroots Nation on Saturday: Sanders and Martin O'Malley. (Hillary Clinton was invited, but declined the invitation.) O'Malley and Sanders made back-to-back appearances in a town hall–style session, moderated by journalist and immigration activist Jose Antonio Vargas.

Both Sanders and O'Malley are trying to win progressive support in the Democratic primary to become the alternative to Hillary Clinton. But only one of them has really succeeded: Sanders's candidacy has gained a lot of momentum, with huge events and the second-most money raised directly of any candidate (driven largely by small donations). O'Malley, on the other hand, hasn't been able to capture as much attention. In fact, he's faced something of an uphill battle with many progressives: O'Malley's political career started in Baltimore, where he was closely associated with the aggressive police tactics that were under protest after the death of Freddie Gray in police custody this spring.

Shortly after O'Malley took the stage, a group of protesters affiliated with the #BlackLivesMatter movement (which has been organizing for the last year or so to call attention to deaths of black men and women at the hands of police) marched into the room chanting "Which Side Are You On?" (a reference to an old-school labor song). Two women (Tia Oso and Patrisse Cullors) took the stage and the microphone and spoke about deaths of black men and women in police custody — specifically the recent suspicious death of Sandra Bland in Texas.

Ending the presentation, Cullors asked O'Malley to offer "concrete actions": "What will you do to stop police unions from battering our names after law enforcement kills us?" "And," she added, "we want to hear it from Bernie Sanders, too."

The activists stayed in the room for the rest of the town hall, chanting the names of people killed in police custody (the hashtag #SayHerName has become a locus for activism and grief after women have been killed in police custody). Neither candidate was exactly graceful in responding to the interruption.

O'Malley, attempting to respond to the protesters, said, "Black lives matter. White lives matter. All lives matter" — a phrase that's been used by critics of the #BlackLivesMatter movement, and which activists see as an attempt to dismiss racial disparities in police shootings. O'Malley later told MSNBC that he wasn't aware of the connotations of "all lives matter," which is itself pretty illustrative of the disconnect between O'Malley and the #BlackLivesMatter movement.

But because O'Malley doesn't have a strong support base among progressives at the moment, he hasn't had supporters step up in his defense. Instead, most people have focused on the response that Sanders — the ostensible progressive champion — gave the protesters.

Why Sanders and his supporters frustrate some racial justice activists

Sanders was defensive and cranky toward the protesters, saying "Black lives of course matter. But I've spent 50 years of my life fighting for civil rights. If you don't want me to be here, that's okay." At other times, he didn't acknowledge the protesters at all and raised his voice to be heard over them (which some attendees saw as Sanders "shouting down" the protesters).

Sanders didn't ignore the issue entirely. But to some observers, it felt like Sanders "stuck to his script" about economic injustice without giving racial injustice its due.

There is a legitimate disconnect between the way Sanders (and many of the economic progressives who support him) see the world, and the way many racial justice progressives see the world. To Bernie Sanders, as I've written, racial inequality is a symptom — but economic inequality is the disease. That's why his responses to unrest in Ferguson and Baltimore have included specific calls for police accountability, but have focused on improving economic opportunity for young African Americans. Sanders presents fixing unemployment as the systemic solution to the problem.

Many racial justice advocates don't see it that way. They see racism as its own systemic problem that has to be addressed on its own terms. They feel that it's important to acknowledge the effects of economic inequality on people of color, but that racial inequality isn't merely a symptom of economic inequality. And, most importantly, they feel that "pivoting" to economic issues can be a way for white progressives to present their agenda as the progressive agenda and shove black progressives, and the issues that matter most to them, to the sidelines.

So Sanders's performance at Netroots confirmed the frustrations that his critics felt. And Sanders's supporters' reaction to the criticism was just as predictable.

Yes, Sanders marched with MLK. But his critics know that already.

Whether you agree with Sanders's claim that he's been "fighting for civil rights for 50 years" depends on whether you think he's doing enough in his Senate career to put civil rights on the agenda. But the "50 years" part is true: He has admirable civil rights movement cred. Sanders was a member of the Student Nonviolent Coordinating Committee, which helped coordinate much of the nonviolent action of the early 1960s, and he participated in the famous March for Jobs and Freedom in 1963, where Martin Luther King Jr. delivered the famous "I Have a Dream" speech.

Today, Sanders's supporters bring up his record in the civil rights movement in response to basically any criticism of his actions on racial equality. And when Sanders started catching criticism for his Netroots performance, the supporters were ready with their history. As comedy podcaster Roderick Morrow — who started the satirical #BernieSoBlack Twitter hashtag on Sunday — told Vox: "It seems like any time black people bring this up on Twitter, there's just all these people who, I don't know if they're just sitting around searching his name on Twitter or something, they just come and get in your mentions and start harassing you, saying the same things over and over to you."

But the civil rights movement references aren't actually an answer to his critics. No one is arguing that Sanders literally doesn't see race, they're saying that Sanders sees racial inequality as less important than economic inequality and shouldn't. And as Vox's Andrew Prokop has written, even in the 1960s, Sanders didn't view race as the fundamental problem many of his fellow student activists did:

Even as a student at the University of Chicago in the 1960s, influenced by the hours he spent in the library stacks reading famous philosophers, he became frustrated with his fellow student activists, who were more interested in race or imperialism than the class struggle. They couldn't see that everything they protested, he later said, was rooted in "an economic system in which the rich controls, to a large degree, the political and economic life of the country."

To Sanders's critics, the "but the civil rights movement!" response isn't just irrelevant, it's insulting. "It's like they're almost trying to outblack us," says Morrow. "'Oh, you're a black person, what could you possibly understand about our candidate? He was marching before you were even born!' That's cool, but you gotta stay on top of it."

So Morrow made a joke:

If I see one more Bernie acolyte mention that he marched with MLK, I'm going to burn the Internet to the ground. @EdDescault @Eclectablog — Imani Gandy (@AngryBlackLady) July 19, 2015

Hold up! Bernie Sanders marched with MLK? This changes EVERYTHING! — Rod TBGWT (@rodimusprime) July 19, 2015

#BernieSoBlack HE teaches you how to Cha Cha Slide! https://t.co/3CJorwWn9b — Rod TBGWT (@rodimusprime) July 19, 2015

Morrow wrote a few more tweets and then went off to record his podcast. When he returned, he saw the hashtag was trending nationwide. "I thought this [harassment from Sanders supporters] was only happening to a few people," he says. "Apparently it was happening to a lot of us."

This isn't about the presidential campaign, it's about the progressive movement

Of course, as always happens on Twitter, some progressive defenders of Sanders — both white and black — used the hashtag to point out that Sanders marched with MLK (thus making critics' point for them); to argue that Sanders' economic agenda would help black Americans; or to contrast Sanders with Clinton. (Others endorsed the #BernieSoBlack jokes but urged people to go after Clinton next.) The latter is a reflection of one of the two main reasons Sanders's supporters are so frustrated by the criticism.

To their minds, Sanders is clearly a more progressive candidate for president than Clinton — so they don't understand why anyone would direct their criticism at the better candidate. After all, Clinton didn't even show up to Netroots.

That isn't how Sanders's critics see it, though. It's worth noting that #BlackLivesMatter organizers haven't been primarily focused on the presidential primary, even as other progressives have turned in that direction. To them, this is about the progressive movement. Bernie Sanders — and, more importantly, the pressure they feel to embrace Bernie Sanders as a progressive champion — is just the latest illustration that some white progressives aren't listening to black progressives when deciding what the "progressive agenda" really is, and who its champions are. If Sanders were polling at 0 percent (like O'Malley) instead of polling in the mid- to high teens, it's unlikely that #BernieSoBlack would have become a popular hashtag. It's exactly because Sanders is being treated as a progressive champion that the activists who challenged him Saturday, and made jokes about his blackness Sunday, feel that, yet again, they're being asked to put their own concerns aside and fall in line with what white progressives want.

And this is why it's not clear whether Sanders can appease his critics. He has spoken about mass incarceration and police violence already. And he does, in fact, have specific policies he's suggested to improve police accountability.

His campaign has even tried to respond to the demands protesters were making Saturday — though there's still something of a learning curve. Morrow told Vox that the campaign had used the #SayHerName hashtag, but named one woman and two men who died in police custody — when the point of the hashtag is to focus on women. (The campaign deleted the tweet.) In a speech in Dallas Sunday, he mentioned Sandra Bland by name. "Thank you for saying her name!" shouted one attendee.

"Maybe I'm being foolish, but, hey, they're hearing. They're receptive," Morrow said of the Sanders campaign. On the other hand, he said, "I'm more hopeful for the campaign than I am for his defenders."

This is a demand on white progressives that goes far beyond Bernie: that they treat racial inequality with the same seriousness that they treat economic inequality. That's not a demand that Bernie Sanders, himself, could fulfill even if he tried. It's a demand on his supporters. And as Morrow points out, it's up against "hundreds of years of history" of people ignoring "a lot of voices, if they don't like what folks are saying. There will always be a struggle, even in progressive spaces. How can you support each other without turning on each other?"