It’s going to be a long campaign season.

Bernie Sanders, the insurgent candidate for the Democratic nomination for President against the frontrunner Clinton, just had his biggest rally so far. In Seattle, 15,000 came to hear him speak, dwarfing numbers for all Republican candidates’ rallies, and for most of Clinton’s.

But a funny thing happened at another rally. He wasn’t allowed to speak. The group Black Lives matter, an activist group that is agitating for police reform and the end to practices that have led to the deaths of scores of unarmed black men and women.

Sanders was the final speaker on a long program held at a city park. Shortly after he took stage, a small group of protesters from a Seattle chapter of Black Lives Matter took the microphone and demanded that the crowd hold Sanders “accountable” for not doing enough, in their view, to address police brutality and other issues on the group’s agenda.

After sharing a few local grievances with the crowd, including school disparities and gentrification in Seattle, the protesters asked for a period of silence to commemorate the one-year anniversary of Michael Brown being shot and killed during a confrontation with a police officer in Ferguson, Mo.

Event organizers allowed the period of silence, as some in the large crowd booed and shouted for the protesters to leave the stage. Afterward, Marissa Janae Johnson, who identified herself as a leader of the Black Lives Matter chapter in Seattle, asked the crowd to “join us now in holding Bernie Sanders accountable for his actions.” She motioned for Sanders to join her at the microphone.

After several minutes of frantic conversations, Sanders left the stage and greeted people in the large crowd who had turned out to see him. Many chanted his name.

Why Bernie? Why haven’t they gone after Clinton or any of the Republican nominees? Possibly because he’s a soft target. His events are open to the public. And on the GOP side, activists can say, “well they won’t listen to us anyway.”

BLM’s tactics have heated up left-leaning political blogs like Daily Kos, where there is a food fight over whether BLM is hurting a candidate thought by many to be a champion of minority rights and police reform, and definitely the most left-leaning candidate in the race. One diary writer is upset with those who are upset at BLM:

The notion that two protests against a single candidate by a non-centralized organization constitute a relevant statistical universe is absurd, but not as absurd as the hysteria that motivates jumping to the conclusion that BLM is a Clinton/Rove plot. It smacks of the same paranoid and patronizing bullshit handed out to ACT UP, all of it at bottom some version of “Behave, Fags, Behave”. I was there, and I remember it well.

I pray Black Lives Matter will keep protesting, keep interrupting, keep standing up, and keep acting up. I stand with Black Lives Matter, inconveniences–to Bernie, Hillary, or anyone else–be damned.

Black Twitter is also unhappy with the BLM protests.

Many Bernie supporters are also ardent supporters of BLM and racial justice. If there are many more interrupted events, there could very well be a rift between the factions.

At any rate, what’s clear is that BLM is determined to make an impact on the 2016 race one way or another.