On Saturday, activists affiliated with the Black Lives Matter movement interrupted a Bernie Sanders rally in Seattle to criticize his campaign for paying insufficient attention to issues of criminal justice and race.

Sound familiar? Something similar happened last month at the progressive conference Netroots Nation: Black Lives Matter protesters interrupted a town hall meeting with Sanders and fellow Democratic presidential candidate Martin O'Malley.

The activists didn't feel that Sanders — and, just as importantly, his supporters — are keeping racial justice front and center. Sanders has become a progressive hero for his economic populism, but at the beginning of his campaign he talked about racial inequality, if at all, as a symptom of economic inequality.

To Black Lives Matter activists and sympathizers, who've spent the last year or more calling attention to the deaths of young black men and women (many at the hands of police), Sanders's attitude toward race was all too familiar: Generations of white progressives have kept economic issues at the center of progressivism and issues that affect mostly nonwhites at the margins. They've challenged Sanders to make racism and mass incarceration as important to his campaign as Social Security.

Sanders's campaign has clashed with activists over their tactics, but it's been receptive to their demands: Sanders is working to show that he's the candidate of all progressives, not just some of them. Yet many of his supporters are tremendously pissed off at the activists for targeting Sanders,who they see as a natural ally of the movement, rather than going after Hillary Clinton or Republicans.

This is really just the latest mutation of an ongoing conflict. Right now, the two sides are Black Lives Matter activists and Bernie Sanders supporters. But white economic progressives and left-leaning activists of color have been struggling over what it means to be a progressive for decades.

Over the past 20 years, both within the Democratic Party and outside of politics, the vision of progressivism that's attracted the most energy and organizing strength has been a progressivism of identity: recognizing the different ways that various groups are marginalized, and working to reduce those disparities both in policy and in everyday interactions. But many progressives in the Democratic Party are inheritors of a labor-liberal progressive tradition that is primarily worried about economic inequality, and are most excited by economic populists like Sanders and Elizabeth Warren.

Sanders supporters see it as obvious that their candidate's platform would be better for people of color than any other candidate's, and they don't understand what else supporters would want. But for the activists challenging Bernie Sanders and his supporters, it's not enough for progressives or Democrats to call for policies that they think would help people of color — they need to be listening to and incorporating the agendas of people of color themselves.

Bernie Sanders's economic populism is at the heart of his appeal

Bernie Sanders has become a surprisingly serious candidate for the Democratic presidential nomination in 2016 by appealing to progressives through economic populism. He's generating tons of excitement; his campaign followed up a 15,000-person rally in Seattle on Saturday with a 28,000-person one in Portland on Sunday.

Sanders was a civil rights activist in the 1960s. But as a politician, he's typically seen racial inequality as a symptom and economic inequality as the disease. The difference between his position and that of other Democratic politicians has become clear over the last year, as politicians and the public have started paying a lot of attention to the deaths of young black men and women at the hands of police or in police custody.

So when Sanders was interviewed by Wolf Blitzer in early May, during the unrest in Baltimore sparked by the death of Freddie Gray in police custody, he emphasized youth unemployment as the "long-run" solution: "In the neighborhood where this gentleman lives [sic], as I understand it, the unemployment rate is over 50 percent, over 50 percent. What we have got to do as a nation is understand that we have got to create millions of jobs, to put people back to work, to make sure that kids are in schools and not in jails."

How have Black Lives Matter activists clashed with Sanders?

In July, Black Lives Matter activists made it clear that they were dissatisfied with Sanders's approach to race during the progressive Netroots Nation conference, when Martin O'Malley and Sanders appeared at a town hall event hosted by immigration activist and journalist Jose Antonio Vargas.

"Your 'progressive' is not enough. We need more."

Protesters interrupted O'Malley, took the stage, and gave speeches about the deaths of young black men and women in police custody — ending with a call for both O'Malley and Sanders to present "concrete actions" for racial justice, and to pay tribute by name to women killed by police or who died in custody.

Sanders was defensive and cranky: "I've spent 50 years of my life fighting for civil rights. If you don't want me to be here, that's okay." The protesters were unimpressed. "Your 'progressive' is not enough," Patrisse Cullors, a co-founder of Black Lives Matter and one of the protesters who took the stage, told the press as a message to Sanders and other presidential candidates. "We need more." The next day, at an event in Houston, Sanders mentioned Sandra Bland (who died in police custody in July) and talked at more length about the issue than he had in the past.

At a Defend Social Security rally in Seattle on Saturday, the pattern repeated itself: Activist Marissa Johnson leaped on stage, approached the microphone, and addressed the audience and Sanders alike. After calling for four and a half minutes of silence for the one-year anniversary (which was Sunday) of the killing of Michael Brown in Ferguson, Missouri, she challenged Sanders again on his lack of a concrete policy to address racial violence — contrasting him with O'Malley, who released a relatively detailed criminal justice platform at the beginning of August.

Sanders stood by silently during Johnson's speech. Attendees weren't so quiet: They booed Johnson, and some called for her arrest. Eventually, according to MSNBC, event organizers made the decision to shut down the event, without Sanders getting a chance to deliver most of his speech.

What do the protesters want from Sanders?

This conflict is playing out on two different levels, and the people who are most upset about what happened in Seattle over the weekend, or what happened at Netroots Nation last month, tend to focus on only one of them.

On one level: Activists are targeting Democratic presidential candidates to ensure that their platforms and campaigns incorporate issues of race and criminal justice. When they've targeted candidates, like at Netroots and in Seattle, they've done so to make particular demands for policy platforms. At Netroots, Cullors demanded, "I want to hear concrete actions. I want to hear an action plan." And in Seattle, Johnson reiterated that request: "Bernie, you were confronted at Netroots by black women [...] you have yet to put out a criminal justice reform package like O’Malley did."

Sanders's campaign has been responsive to protesters; many of Sanders's supporters have not

Their tactics may be unorthodox, but the dynamic is pretty typical of the relationship between "outsider" activist groups and candidates during primary elections. Activists aren't exactly challenging candidates to earn their votes; they're saying that in the year 2016 it should be a requirement for any Democratic candidate to discuss issues of race and criminal justice, and challenging candidates to meet that minimum.

And on that level, things are going pretty smoothly. Since the confrontation last month at Netroots, Sanders and his campaign have clearly been working hard to meet activists' demands. At a rally in Houston the day after Netroots, Sanders addressed the death of Sandra Bland in police custody. On Sunday night, after the Seattle event, Sanders's campaign released a draft platform for racial justice, which addressed mass incarceration, policing, and voting rights as well as economic issues.

Activists are also targeting Sanders as a way to target Sanders supporters

Bernie Sanders's campaign has, for the most part, been responsive to protesters and critics. Many of Bernie Sanders's supporters — especially on social media — have not. As Roderick Morrow, a podcaster and Twitter personality who started a joke #BernieSoBlack hashtag on Twitter after the Netroots confrontation, explained to me in July:

[T]here's all these people who, I don't know, they're just sitting around searching his name on Twitter or something, they just come and get in your mentions and start harassing you, they start saying the same things over and over to you[...] it's almost as if they're trying to say, "You shouldn't expect him to continue this" or, "Because he's done stuff in the past, you shouldn't question him now." I thought it was happening to just a few people — apparently it's happening to a lot of us.

Some Sanders supporters attending the Seattle event booed the protesters taking over the stage, and some even called for them to be arrested (which, as Black Lives Matter supporters point out, would be exactly the kind of aggressive policing they're trying to wake supporters up to). And after Seattle, some progressive media outlets are getting frustrated with the continued challenges to Sanders. Gawker's Hamilton Nolan wrote Monday that the activists going after Sanders are "pissing on their best friend."

This is the second level that protesters are attempting to address by disrupting Sanders: to get through to his fans.

Morrow and others see this as a continuation of a longstanding dynamic within the self-identified "progressive movement." White progressives can often ignore issues that disproportionately affect people of color (like mass incarceration), or treat them as secondary to "real" issues like economic inequality. But as progressives, they think of themselves as defenders of people of color and other marginalized groups — so when they're challenged on their anti-racist bona fides by activists of color, they tend to react with disbelief, defensiveness, or outright hostility.

That's exactly how the drama played out in Seattle. When Sanders took the mic, he thanked Seattle for being "one of the most progressive cities in the United States." When protester Marissa Johnson took it, she responded to the crowd's boos by saying, "I was going to tell Bernie how racist this city is, even with all of these progressives, but you’ve already done that for me."

Identity-based progressivism is ascendant in American culture, but economics are still the heart of progressive politics

In some ways, Bernie Sanders is a throwback to an older version of the Democratic Party: one where labor groups were much more powerful within the party than they are today, and identity-based interest groups were much less powerful. The Democratic power structure has embraced diversity as a goal over the last 20 years.

At the same time — especially over the past few years — identity-based progressivism has been ascendant in American culture. The Black Lives Matter movement has gotten tremendous amounts of attention over the past year. College campuses are working (with mixed success) to crack down on sexual assault in response to progressive demands. Corporations are showing off their support for same-sex marriage on social media; Caitlyn Jenner's coming out as transgender got her an award from ESPN for courage. And many of these changes are the result of activists targeting corporations, media companies, and other entities in the same way they'd target politicians. (Just think of the campaigns earlier this summer asking companies to cut ties with Donald Trump over his comments about Mexican immigrants.)

But the nexus of these two — progressive politics within the Democratic Party — is something of an exception to these trends. Many progressive voters are deeply worried about economic inequality, and about the domination of both the economy and politics by the superrich. To their minds, this is the existential crisis facing the country. Before the presidential election, the foremost progressive champion in Democratic politics was Massachusetts Sen. Elizabeth Warren, whose entire political career has been built on taking on the financial industry. And Sanders is now generating Warren-like levels of excitement for his outspoken socialism. Remember, the rally he was holding in Seattle — during the weekend that marked the one-year anniversary of the death of Michael Brown in Ferguson — was about defending Social Security. That doesn't mean Sanders or his supporters didn't care about Ferguson, of course, but it is a choice of emphasis.

The Democratic progressives rallying around Warren and Sanders may agree that racial or gender inequality is also a problem, but they may see it (as Sanders long did) as a problem that can best be solved by fixing economic inequality. Or they may see them as issues that politicians should address, but not necessarily ones they need to focus on. To nonwhite progressives, especially activists, this makes it feel like "progressivism" is still something for white people.

But why is shutting down a Bernie Sanders rally the best way to accomplish this?

The criticism of the Seattle protesters, in particular, is largely about their tactics: that they could have picked a better way to get their message across than taking the mic away from Bernie Sanders. By being so confrontational, the criticism goes, they alienated the rally attendees who would be their natural allies.

Activists get hit with this criticism whenever they engage in tactics that disrupt other people, from blocking traffic on public roads to holding "die-ins" at New York City restaurants during weekend brunch. But as they often point out, when they try to get their message out using less disruptive tactics, they're often ignored by the same people who criticize disruption.

This comes back to the idea that activists are trying to propagate, that fully incorporating racial justice into a campaign is the least that any Democrat should be expected to do in 2016. The Black Lives Matter movement has been in the public eye for a full year. If that hasn't inspired candidates to embrace racial justice as fully as activists might like, activists assume it's unlikely that they're going to do more on their own, without a challenge.

Why isn't this happening to Hillary Clinton?

One of the main lines of criticism leveled at the activists who've interrupted Sanders events is that they're not putting Hillary Clinton up to the same level of scrutiny. Some Sanders fans allege that the protesters are deliberately "in the tank" for Clinton; others don't go that far, but worry the result of targeting Sanders will be that Clinton, whom they view as an objectively inferior candidate, will win the nomination.

That criticism makes less sense once you view the challenges to Sanders as a challenge to Sanders's fans as well. The entire reason that Bernie Sanders has become a surprisingly relevant candidate in the 2016 Democratic primary is that he's excited a share of the white progressive base that wasn't in any way excited about Clinton. Sanders's campaign is less likely to send out a press release about a poll result or (heaven forbid) a fundraising total than they are to send out a press release about how many people Sanders drew to a rally. And those rally attendees — the people who are excited about Bernie Sanders — are, in large part, the white progressives that have frustrated activists of color for so long.

But when thinking about protesters' other goal — making it a minimum requirement for any Democratic candidate to incorporate issues of race, racial violence, and criminal justice fully into their campaign — the "What about Hillary?" question makes a lot more sense. But there's actually a perfectly nonconspiratorial, even obvious reason that Hillary Clinton campaign events aren't getting disrupted: Hillary Clinton isn't holding disruptable campaign events. She's holding small events with voters in early primary states, or massively stage-managed rallies — and always with a Secret Service detail at the ready.

In fact, on August 11, Black Lives Matter activists planned to disrupt a Clinton town hall in Keene, NH — but were locked out of the venue, which the Secret Service said was at capacity.

Organizing Bernie Sanders might be the best way to organize Hillary Clinton

The difference isn't just happenstance; it's the result of the difference between the campaign Hillary Clinton is running and the campaign Bernie Sanders is running (and that Martin O'Malley appears to hope to run). Clinton is the frontrunner for the Democratic nomination, and is currently working to shore up her support throughout the Democratic Party. She doesn't need progressive voters to love her; she just needs them to accept, ultimately, that she's the best woman for the job. Her challengers, on the other hand — especially Sanders — are only relevant in the race to the extent that they're generating more enthusiasm than Clinton is.

But the Clinton campaign appears to believe that it can't afford to ignore these issues entirely. Clinton's first major policy speech of her campaign was on criminal justice; after the Netroots Nation confrontation, she made a point during a Facebook Q&A of answering a question from the Washington Post's Wesley Lowery about how she "would have" responded.

The way Clinton's running her campaign, putting more distance between the candidate and activists than Sanders is, gives her the luxury of a little more time to craft a message. But it seems her campaign doesn't want to be in the position where they're lagging behind other candidates on the issue. Some Black Lives Matter organizers are already saying that now that both Sanders and O'Malley have released policy documents about criminal justice reform, there will be more pressure on Clinton to join them. In that respect, the protesters targeting Bernie Sanders are engaged in a third organizing tactic: organizing Sanders as a way to get to Clinton.

Watch: What Bernie Sanders said about race in the Vox interview (@ 16:05)