(CNN) Bernie Sanders is all the rage again.

After sputtering in most polls, the breakout star of the summer has significantly narrowed the gap in national match-ups against Hillary Clinton, expanded his lead in New Hampshire and overtaken the former secretary of state in Iowa.

His surge, supporters say, evokes flashbacks of 2008, when Clinton underestimated then-Sen. Barack Obama's strength and lost the nomination in a prolonged and nasty contest. Like Obama, Sanders has sparked a movement powered by young voters who see the septuagenarian not just as a protest candidate, but as the future of the progressive movement. And his $75 million fundraising haul guarantees his viability.

But for all the good news for Sanders, he still faces the same electability question: can a Democratic socialist from the second-whitest state in the country win voters that actually look like the rest of the Democratic Party? After Iowa and New Hampshire -- two overwhelmingly white states -- Sanders faces an electorate that is much more diverse and not as familiar with Sanders, especially in the South, which at the moment is Clinton Country.

If he can't expand his base, Sanders can tout all the polls he wants that show him doing better than Clinton in hypothetical head-to-head contests with Republicans, but he won't be the Democratic nominee.

It's an issue he will have to address beginning at Sunday night's Democratic presidential debate in Charleston, South Carolina.

Clinton has been out ahead of Sanders in courting black voters, touting this week an endorsement from Obama's former attorney general, Eric Holder, who said that Clinton is the best candidate to build on Obama's legacy. He will appear with the former secretary of state in an upcoming swing in South Carolina.

Clinton supporters say that her team understands the bloc of voters that drives the overall black vote. "The key demographic that matters is African-American women and Karen Finney and LaDavia Drane and Maya Harris are very much focused on that group," said Bakari Sellers, a former state legislator in South Carolina and CNN contributor, listing Clinton's top aides. "They are going to message and organize and not just win it, but lock it down overwhelmingly."

Sanders will be featured in next month's issue of Ebony magazine and on Sunday in a debate in South Carolina sponsored by the Congressional Black Caucus Institute, he will have a chance to connect with black voters and post up against the former secretary of state on issues like criminal justice reform, education, and the economy. Monday, he will appear at a King Day rally in Birmingham, Alabama, with Cornel West and Nina Turner, a former Ohio legislator who made waves when she jumped from Clinton to Sanders last fall.

"It's about six months of communication versus 20 years," said Marcus Ferrell, who is Sanders' African-American Outreach Director, comparing Sanders to Clinton. "For us to be where we are is good. It's not like our numbers have gone down, they have crept up."

The campaign also plans to release a series of ads aimed at introducing Sanders to South Carolina voters.

"The case we're making is 'Listen, this guy is a stronger candidate than Hillary Clinton in electoral politics,'" said Tad Devine, a senior Sanders strategist. "We have objective measurements of that -- it begins when voters start choosing."

A national New York Times/CBS News poll shows Clinton with the support of 48% of Democratic voters, and Sanders closing in with 41%. But South Carolina, Nevada and the southern "SEC primary" states all pose electoral challenges for the senator.

Clinton won Nevada on the strength of Latino voters in 2008, yet faltered among black voters across the South who backed Obama by 80%, after he proved that he could win white voters in Iowa.

Her path to victory is built on winning the big states she won in 2008, doing well with Latino voters and co-opting Obama's playbook among black voters. Beyond winning states that look like Vermont, Sanders' long-term playbook is much less clear. Aides seemed less confident about an upset in South Carolina, but bullish on Nevada.

"Our commitment in Nevada is not symbolic, it's real. Bernie's been there many times and will be there again," Devine said. "So we're trying to organize a winning caucus campaign in Nevada. We're doing that by pouring in resources."

Chasing black voters

Thursday, the Sanders campaign launched its most robust outreach effort among black voters, kicking off a tour of historically black colleges at South Carolina State University in Orangeburg.

Sanders wasn't in attendance, but frequent surrogate West was. West told students that Sanders marched with Martin Luther King in 1964, "when it was not popular, it didn't win any moral prizes," adding, "He did it because it was right and moral and just. That is why he was with Brother (Jesse) Jackson." Rapper Killer Mike, who hosted Sanders in his Atlanta barbershop for an interview, will also be on hand for parts of the tour. The campaign is expected to name other high profile surrogates in the coming days.

"No other candidate has made such a concerted effort to reach out to HBCUs," said Symone Sanders, a press aide for the Sanders campaign. "We are taking the message right to the people."

Photos: State of the Union 2016 Photos: State of the Union 2016 President Barack Obama delivers his final State of the Union address before a joint session of Congress on Capitol Hill on Tuesday, January 12. Hide Caption 1 of 16 Photos: State of the Union 2016 Obama speaks to the joint session. Hide Caption 2 of 16 Photos: State of the Union 2016 Obama reads from the text of his State of the Union address. Hide Caption 3 of 16 Photos: State of the Union 2016 Vice President Joe Biden and House Speaker Paul Ryan listen as Obama delivers his address. Hide Caption 4 of 16 Photos: State of the Union 2016 Biden points at Obama during the State of the Union. Hide Caption 5 of 16 Photos: State of the Union 2016 Members of Congress, including Republican presidential candidate Sen. Marco Rubio, center, listen. Hide Caption 6 of 16 Photos: State of the Union 2016 Obama gives copies of his speech to Biden and Ryan. Hide Caption 7 of 16 Photos: State of the Union 2016 First lady Michelle Obama sits beside an empty chair as her husband speaks. The chair represents the victims of gun violence in America. Hide Caption 8 of 16 Photos: State of the Union 2016 Obama gestures to an attendee as he arrives. Hide Caption 9 of 16 Photos: State of the Union 2016 Satya Nadella, CEO of Microsoft, center, claps as people enter the chamber. Hide Caption 10 of 16 Photos: State of the Union 2016 Obama greets former Arizona Rep. Gabrielle Giffords. Hide Caption 11 of 16 Photos: State of the Union 2016 Supreme Court Justice Elena Kagan, from left, Chief Justice John Roberts, Justice Anthony Kennedy, Justice Ruth Bader Ginsburg, Justice Stephen Breyer and Justice Sonia Sotomayor wait for Obama to arrive. Hide Caption 12 of 16 Photos: State of the Union 2016 Tuesday's address before a packed House chamber marked the debut of new House Speaker Ryan -- a longtime Obama rival -- on the platform alongside Vice President Biden.

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A Fox News poll in early December showed Clinton with 82% of the black vote in South Carolina and Sanders drawing 11%. Aides have floated a goal of winning a third of the black vote there.

Last month, the Vermont senator traveled to West Baltimore, the site of the Freddie Gray arrest and subsequent unrest last year and toured one of the city's poorest neighborhoods with black pastors. After his hour-long tour of Baltimore's Sandtown neighborhood, dotted with vacant and boarded-up rowhomes, Sanders sat for a roundtable with the clergy members.

They praised him in a news conference afterwards, but also made a point of saying they were not endorsing Sanders.

After Sanders left the Baltimore meeting, Turner rallied the assembled clergy, saying the Democratic Party had treated the African-American community like its "mistress" for too long. She then invited them onto a conference call with Sanders the next day.

"He has a righteous indignation, the same fire in the belly that I have," Turner said in an interview with CNN. "When he came to Cleveland State University he said to a majority white crowd that as the next President he would work to eradicate institutional racism. He has a boldness that the party needs."

Electability questions dog Sanders

Turner acknowledged that Sanders has a hurdle in appealing to black voters throughout the South, a more socially and politically conservative region where protest votes are less likely. In 2004, black voters in South Carolina largely rejected Al Sharpton's candidacy, voting for John Kerry and John Edwards in larger numbers. The electability argument, is especially important to make to black voters, Turner said.

Sanders himself delivered a version of his electability pitch to reporters on Thursday.

"Republicans win when voter turnout is low, Democrats and progressives win when voter turnout is high. One of the reasons I believe I am the strongest candidate for the Democratic nomination, in the general election, is that I believe in the energy and the enthusiasm that we are seeing in this campaign will result in a large voter turnout in November and in victory," Sanders said.

But Rick Wade, Obama's 2008 black vote director, said the Vermont senator may simply be too late.

"It's now an issue of timing for Sanders," Wade said. "This should have started two years ago -- building infrastructure and relationships across South Carolina and beyond. "He is resonating in terms of the issues, but people have to know you and feel you. It is different when you have a black man named Barack Obama."