Vermont Sen. Bernie Sanders booked his biggest speaking gig to date, drawing some 11,000 activists to an overflow room at the Phoenix Convention Center. Sanders speech fortuitously coincided with the annual gathering of “NetRoots Nation,” a national meeting of thousands of leftist activists, bloggers and special interests.

The presence of so many leftist activists in town for the conference no doubt spiked attendance for Sanders speech. Its hard to imagine an aging hippy ticking through a list of left-wing policy issues attracting anyone but the most addicted political junkie.

That said, Sanders’ draw and the overall combative tone of the NetRoots conference presents an emerging challenge for Democrat party regulars.

Earlier in the day, both Sanders and former Maryland Gov. Martin O’Malley were targets of a protest during a town-hall style panel at the conference.

O’Malley, who spoke first on the panel, was interrupted by around 40 “Black Lives Matter” protesters, who stormed the room while an activist seized the microphone.

“We are going to hold this space and acknowledge the names of black women who have died in police custody, and then Governor O’Malley we do have questions for you!” protester Tia Oso said. “As leader of this country will you advance an agenda that will dismantle structural racism in this country?”

Both O’Malley and Sanders, who wear leftist politics on their sleeve, seemed taken back by the exchange. In part of his response, O’Malley said that “all lives matter,” a tautology that he was forced to apologize for later. He said he was sorry if that phrase “offended” anyone.

Sanders, amid of torrent of jeers, turned to the moderator and asked “What are you doing here?” and “Are you in charge here?”

The twin events on Saturday show the promise and pitfalls of the growing agitation among the hard-core left. On the one hand, their attendance helped fuel Sanders’ largest campaign event to date, providing an important backdrop for the media coverage that has powered his rise in the polls.

On the other hand, though, the palpable anger among these most committed activists, stoked over the years by the explicit design of President Obama and national Democrats, is taking over the party. It is driving the national party not only into the far fringes of leftist philosophy, but also a deep embrace of identify politics that is likely to alienate most voters.

When “all lives matter” is on offensive statement, virtually everything is offensive.

As one Black Lives Matter protester explained later, “It’s not like we like shutting shit down, but we have to. We are tired of being interrupted.”