Over the weekend, Sen. Bernie Sanders, I-Vt., was in the key early primary state of South Carolina. He's been drawing huge crowds around the country in his campaign for the Democratic nomination, but even if he succeeds elsewhere, South Carolina could be a big hurdle.

If Sanders has any hope of beating Hillary Clinton in South Carolina, he'll need black voters. They make up most of the state's Democratic base. But even in the predominantly African-American city of North Charleston, the crowd that showed up to see Sanders was mostly white.

In a July ABC News/Washington Post poll, Sanders trailed among white Democratic voters with 22 percent, versus 58 percent for Clinton. Among nonwhites, Sanders dropped to 9 percent, while Clinton rose to 71 percent.

His struggle to connect with black voters has been amplified as Black Lives Matter activists have disrupted a couple of his events in other states.

In North Charleston this weekend, Sanders argued he's the candidate most concerned about racial justice.

"Let me be very clear," Sanders said. "Nobody will fight harder to end institutional racism and to reform our broken criminal justice system."

Black Lives Matter leaders from the Charleston area attended the speech but did not disrupt it. Several, including Muhiyidin d'Baha of North Charleston, met with Sanders afterward.

"Bernie's going through his own evolution," d'Baha said. "Coming from Vermont and being in the space of white privilege that he's enjoyed, he's gonna go through an experience of learning."

At an event in Columbia, Juanita Moore was among the relatively few African-Americans in the crowd. Moore likes what Sanders is saying about health care and poverty, and she's concerned about the recent headlines about Clinton's use of a private email server while she was secretary of state.

"To me, if you're gonna be a candidate in this country, you need to start with a clean slate," Moore said.

A big question is whether Sanders can persuade more African-Americans to support him, says Danielle Vinson, a political scientist at Furman University. She calls Sanders an "odd choice" for South Carolina Democrats, who tend to be relatively conservative.

"He would play well in parts of the West Coast," Vinson said. "But can you reach Southern Democrats? Can you reach African-American voters?"

Chris Covert, the state director for Sanders in South Carolina, says the campaign has "a ton" of work to do. He acknowledges that winning over Clinton supporters in South Carolina will be a battle. She lost the state to Barack Obama in 2008, but the Clintons otherwise have a long history of support from black voters.

"They've been around for a long time," Covert said. "But Bernie's been around fighting for civil rights for 30 years; he's been around fighting for Medicaid [expansion] for 30 years. He's been around fighting for jobs for a long time. He has a great base here, too."

Another question hanging over the Democratic race is whether Vice President Joe Biden will jump in. He recently vacationed in South Carolina, where he was said to be contemplating whether to launch a campaign. In recent days, it's been widely reported that he is more actively exploring a possible run.

Scott Huffmon, a political scientist at Winthrop University, runs the Winthrop Poll in South Carolina. He says Biden would "really upset" the race, but it's not clear precisely how. Under one scenario, Huffmon says, Biden could siphon off some of the activist support for Sanders. Then there are Clinton supporters who may be nervous about the email controversy.

"If Biden enters the race, he is also going to peel off a lot of support from Hillary," Huffmon said. "So it would really kind of upend things in South Carolina in the short-term."

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