While much of the political world obsessed about the latest developments in Donald Trump's takeover of the Republican presidential race, an extraordinary thing happened on the Democratic side. In Seattle on Saturday, the most liberal candidate in the Democratic contest, Sen. Bernie Sanders, I-Vt., was shouted down by Black Lives Matter protesters, who took the event at which he was scheduled to speak hostage.

It was an ugly moment; the anger and bad feelings onstage were intense, judging from several videos of the event. A tiny number of protesters seized control of the stage, sending an intimidated Sanders into full retreat. No one could watch the proceedings without concluding Sanders has a serious problem on his hands.

The occasion was an afternoon "Social Security Works" rally in Seattle's Westlake Park. The outdoor event drew a few thousand people. (Sanders' big Seattle rally, attended by 15,000, was Saturday night.) As Sanders approached the microphone, protesters began chanting, "Black lives matter! Black lives matter!"

Sanders had barely gotten out his first line — "Thank you Seattle for being one of the most progressive cities in the United States of America!" — when two women approached the podium. Sanders actually reached out to shake hands, but that's not why they were there.

"We're shutting this event down now," Marissa Janae Johnson, a co-founder of Black Lives Matter Seattle, said to Sanders.

"If you do not listen to her, your event will be shut down right now," said Mara Jacqueline Willaford, another co-founder. "Right now!"

The crowd figured out what was going on. "Let Bernie talk!" some chanted.

Event organizer Robby Stern intervened, as Sanders moved a couple of feet away. "We are trying to be reasonable," Stern said as Johnson and Willaford yelled at him. "We're going to let you on the mic, we are going to give you the mic — after Sen. Sanders."

"Stop talking!" Willaford screamed.

"After Sen. Sanders — "

"Let her speak!" Willaford screamed, pointing at Johnson.

"After Sen. Sanders — "

"Let her speak!"

"After Sen. Sanders — "

"LET HER SPEAK!"

"Alright, we're shutting it down," said Stern. "You are disrupting this event."

The crowd chanted. "Bernie! Bernie! Bernie!"

Sanders approached Johnson. "Hold it, ma'am," he said. "I'm happy to — Hold it! Excuse me — "

"Don't take a tone with me," said Johnson.

"If that's your attitude, then I have nothing to say," said Sanders.

Meanwhile, the organizers were caving fast. "How long do you want?" asked Stern.

"Stop asking questions and let her speak now!" yelled Willaford.

Stern took the mic. "We have been saying all along that we're fully supportive of the Black Lives Matter movement," he told the crowd. "We are going to let them have a little time at the mic. I'm asking you: Please let them speak."

Sanders took a few steps to the side. The crowd booed. And the haranguing began in earnest.

"You guys are full of bull---t with your 'black lives matter,'" Johnson told the overwhelmingly white crowd. "You're never going to hear Bernie speak unless I hear silence here now."

More booing.

"I was going to tell Bernie how racist this city is, filled with its progressives, but you already did it for me," Johnson said. "Thank you. Now that you've covered yourself in your white supremacist liberalism, I will formally welcome Bernie Sanders to Seattle."

As she spoke, Sanders stood nearby, helpless, as Johnson proceeded to condemn Seattle, where officials are building "a new jail to imprison black children," where the police practice racial profiling, where the schools suspend black students more than whites, and where there has been "intense gentrification."

And then, noting the next day would be the first anniversary of the "ruthless murder" of Michael Brown in Ferguson, Mo., Johnson let everybody have it:

"So right now we're going to honor this space, and we're going to honor the memory of Michael Brown, and we're going to honor all of the black lives lost this year, and we're going to honor the fact that I have to fight through all these people to say MY LIFE MATTERS! That I have to get up here in front of a bunch of screaming, white racists to say my life f---ing matters!"

That didn't go over too well with the progressive audience. How could she say that about them? But Johnson went on, demanding four and a half minutes of silence, "which is representative of the four and a half hours [Brown's] body lay in the street."

Sanders finally surrendered completely and left the stage. Moving into the crowd to shake hands, he told a reporter the hijacking of the rally was "unfortunate, because among other things, I wanted to talk about the issue of black lives." A lifelong liberal who took part in the 1963 March on Washington, Sanders is trying desperately to mollify the Black Lives Matter groups, but so far, nothing he has said or done has been enough.

Why put up with the harassment? Because for Democrats, having alienated growing numbers of white voters, black votes matter now more than ever.

"It's tough to overstate just how critical black voters have become to today's Democratic coalition, particularly when it comes to the Electoral College," note Amy Walter and David Wasserman in a recent study from the Cook Political Report.

"Deconstructing exit poll data from 2012, African-American voters accounted for Obama's entire margin of victory in seven states: Florida, Maryland, Michigan, Nevada, Ohio, Pennsylvania and Virginia. Without these states' 112 electoral votes, Obama would have lost decisively." (Hispanic voters, many clustered in states that are already Democratic, were not nearly as important to the Democratic candidate.)

So Sanders stepped back and gave the protesters the microphone. And probably not for the last time.