(CNN) In his bid to ride a progressive wave to the White House, Bernie Sanders is failing to connect with a key Democratic constituency: African-Americans.

Democrats are pinning their electoral fortunes on African-American and Latino voters. But the Sanders revolution looks a lot like Vermont, the second whitest state in the country. To mount a competitive challenge against Hillary Clinton, Sanders must do something he has never had to do—reach beyond the kind of post-racial political message he honed in his home state and connect with voters who don't look like him.

And so far, he's coming up short.

"I haven't seen him engaging the black community. Nor am I hearing any chatter about him," said Rick Wade, Obama for America's African-American vote director. "Black voters don't know him."

A June CNN/ORC poll showed just 2% of black Democrats supporting Sanders, a figure that has remained unchanged since February. Among non-white voters overall, Sanders polls at 9% compared to Hillary Clinton's 61%.

The challenge was on full display at this weekend's Netroots Nation conference, a gathering of thousands of left-leaning Democrats. With his populist, soak-the-rich rhetoric, this was supposed to be prime Sanders territory.

Shaky response

A group of protestors crashed his speech , forcing Sanders to go off script while they chanted "black lives matter," a phrase that has become the rallying cry for a new civil rights movement.

His initial response was shaky and some activists found Sanders dismissive.

"Black lives, of course, matter. I spent 50 years of my life fighting for civil rights and for dignity," Sanders said. "But if you don't want me to be here, that's OK. I don't want to out-scream people."

After the protests, Sanders retreated from public view for several hours. He canceled a small group meeting that included Elon White, a host of a show about black issues, and he was a no-show at an event hosted by the Arizona Democratic Party. Meanwhile, former Maryland Gov. Martin O'Malley, who is also mounting a liberal challenge to Clinton, participated in panels at the conference and took question from on the online interview show, "This Week in Blackness."

Sanders re-emerged later that evening for a short speech at a private fundraiser for the Latino Victory Project, an advocacy group led by major Democratic donors that focuses on Latino outreach, where he, again, took questions about race.

But instead of adopting the defensive stance from that morning, Sanders' tone changed. He appeared humble, and asked questions of the activists gathered about how he could better pursue and talk about policies to combat racial injustice.

"I want some help on this. I'm being very honest," Sanders said. "I want some ideas, as somebody who was arrested 50 years ago fighting for Civil Rights trying to desegregate schools in Chicago, who spent his whole life fighting against racism, I want your ideas. What do you think we can do? What can we do?"

Sanders took on questions about what activists called "institutional racism" in education, housing, and criminal justice, and called for changes in leadership and an end to mass incarceration. He listened to ideas from the small group, and he even brought those who questioned him to stand next time while he spoke. He said that stories of minorities being shot by police was "horrendous" and "unacceptable," but touted progress that had been made since he began first started protesting for civil rights as a young radical in the 1960s.

'Impatient'

"I wish I could tell you I had a magical formula for how to end racism. It's gone on in this country for before it was America," Sanders said. "Let me just say this, because I'm older than you: You're impatient, and you should be impatient. But if we were standing here 30 years ago and somebody said, 'You know, I think in 2008, the United States of America will elect an African-American president,' you know what people in this room would've said? They would've said, 'You are crazy. That will never happen because America has too much racist baggage.' But it happened. So, I guess you're right to be impatient, but we have made progress in this country. Not enough."

Sanders and his aides have long insisted that his central message of fixing income inequality, providing universal health care and raising the minimum wage will help all Americans -- Latinos and African-Americans most of all.

In his campaign announcement and in subsequent speeches, he bashes oligarchs and vows to take on the "billionaire class."

But in most of his speeches, he is largely silent on immigration, racial inequality, police brutality and criminal justice reform, all front-bunner issues for the sizeable African-American voting bloc.

Sanders, who has narrowed Clinton's lead in states like Iowa and New Hampshire, is trying to broaden his reach, but his stumble at Netroots suggests that, for Sanders, expanding beyond class to matters of race will be a challenge.

"He is not a rainbow coalition guy or at least he hasn't been," said Greg Guma, author of "The People's Republic: Vermont and the Sanders Revolution," who has known Sanders since the 1970s. "He feels like he knows what the problem is and it's monopoly capitalism. Anything that takes him away from that message is a distraction."

He has opportunities to connect more deeply with African-Americans in the days ahead. He is slated to speak Saturday at the Southern Christian Leadership Conference in Baton Rouge and at the end of the month he is set to address the National Urban League in Fort Lauderdale. A campaign aide said that Sanders will talk about race, touching on malnutrition in the inner city and youth unemployment among other topics in upcoming speeches.

Last week Sanders, 73, spoke at the National Council of La Raza, where he touted his immigrant roots and vowed to push for a pathway to citizenship.

'Energize minorities'

And he recently tweeted: "We must energize minorities all across the country to engage in the Democratic process and thwart efforts to disenfranchise minority voters."

"It's important for him to make sure the core constituencies know who he is," said Tad Devine, a senior campaign adviser. "We have a story to tell about him and we are going to campaign actively in different communities."

Campaign trips to Mississippi, Alabama, and South Carolina are planned for August—all three states have significant black populations.

Spotlighting Sanders' early activism will be key to connecting with African-American voters, Devine said.

His campaign biography now includes a reference to his participation in the March on Washington. His aides often mention his endorsement of Jesse Jackson in the 1988 Democratic primary, a move that helped the civil rights leader win Vermont. Sanders also joined the Congress of Racial Equality (CORE), as a student at the University of Chicago.

In an ironic twist, Sanders' civil rights activism stretches further back than Clinton. While she was a "Goldwater girl," Sanders was paying a $25 fine for leading a protest march against school segregation in Chicago.

But, to some, Sanders' civil rights credentials seem dated.

"I'm happy to know that he was rallying in the 1960s. That tells me he was on the path when I was a child, but not too many people are thinking about the 60s" said Mary Brown-Guillory, president of the Vermont chapter of the NAACP. "What is he doing today about police brutality and the right to vote and equality? He has to be more than just Vermont."

Sanders' home state is 95% white. The NAACP held its first-ever state chapter meeting earlier this month.

Though Sanders' run and his ability to draw huge crowds has brought comparisons to Obama's 2008 bid, the more apt comparison is to Howard Dean, who also struggled to gain traction outside of Vermont in 2004.

But aides suggest that Sanders has a better story to tell, with his past activism and current political resume as a key selling point.

He has a solid 100% in the most recent NAACP scorecard. As a candidate for mayor of Burlington in 1981, Sanders campaigned against a plan to convert Burlington's Lake Champlain waterfront into an enclave for the rich. He announced his bid for the White House on the shores of Lake Champlain, highlighting his fight for the underdog and a class battle that he managed to win.

But absent from his announcement were people like Curtiss Reed, the executive director for Vermont Partnership for Fairness and Diversity, who skipped the event because of Sanders' one-note emphasis on class.

'Remarkably consistent'

"His message is remarkably consistent in that it is devoid of any conversation around race. He is colorblind to an extent that it seems that race is something that is uncomfortable for him to talk about. He is like a lot of Vermonters," Reed said. "It's easy to rattle off statistics, but that's not engaging people of color."

Clinton, whose weakness among black voters in 2008 cost her the nomination, has made a concerted effort to try to lock-up the Obama coalition. She gave a race speech after the Charleston massacre, called for an end to mass incarceration in her first policy speech and has made two campaign stops in South Carolina. She met with the Congressional Black Caucus last week as well as the Congressional Hispanic Caucus, mindful that there are delegates to be had among party leaders.

But even Clinton, who saw the intersection of race and class up close in Arkansas, has struggled to keep up with the rhetoric of young, progressive blacks. In a June speech at a black church near Ferguson, Missouri, she said "all lives matter," rather than "black lives matter." Grassroots activists, who see such generalizations as a way to sidestep specific race-based remedies, were outraged.

Donna Brazile, a CNN contributor and longtime Clinton friend, pointed out on Twitter that Clinton has said black lives matter in the past.

Yet those are the kind of ties and connections that Sanders lacks. Several grassroots activists in Vermont complained that he didn't reach out as he ramped up his national campaign. And when they look at his campaign team, they see the Vermont problem.

"If he is going to have a serious national campaign, he is going to have to better reflect the changing demographics with his team. Diversity is a strength," said Hal Colston, a black Sanders supporter who runs Partnership for Change, a non-profit education group in Vermont. "Bernie gets it and he does listen. He's always been the champion for the person left behind. He knows he has to update his rhetoric and he is already showing signs of expanding the conversation."

Updating the conversation will mean responding to a rapid social media driven movement that is now trying to bring attention the the case of Sandra Bland, a Texas woman who died in police custody. Campaign aides said that in the coming months, they will bring on more staff, including people of color steeped in policy issues. For her part, Clinton has scooped up black policy advisers as part of her core team, putting Sanders at somewhat of a disadvantage.

"It will take some time because we are not the frontrunner, but it's a priority," Devine said. "We want a campaign that looks like a Democratic party campaign and not a Republican party convention."

Henderson reported from Washington while Moody reported from Phoenix.

CORRECTION: A previous version of this story identified Donna Brazile, a vice-chair of the Democratic National Committee, as a supporter of Hillary Clinton. She has not endorsed any candidate in the Democratic primary race.