As Hillary Clinton scrambles to stop the momentum of Bernie Sanders in the Democratic presidential primary, her allies are ramping up efforts to portray the Vermont senator as unacceptable to the black voters she must win.

The most blatant example of the race card being played came after Sanders released his television ad to the Simon and Garfunkel song “America” this week.

David Brock, the liberal pro-Clinton attack dog, accused the Sanders campaign of including too many white people in the ad. He called it “bizarre” and a “significant slight to the Democratic base.”

“From this ad it seems black lives don’t matter much to Bernie Sanders,” Brock told the Associated Press.

The strategy to portray Sanders as unacceptable to black people makes political sense for Clinton. Polls are showing Sanders capable of winning both Iowa and New Hampshire and Clinton would need to maintain the support of black voters, especially in southern states like South Carolina, to bounce back.

Sanders, a self-proclaimed Democratic socialist, is popular with young, white liberals but has struggled with black voters. The Clinton campaign needs to keep it that way.

[dcquiz] During the recent Democratic debate in Charleston, hosted by the Congressional Black Caucus Institute, Clinton accused Sanders of attacking President Obama.

“Sen. Sanders called him weak, disappointing,” Clinton said during the debate. “He even, in 2011, publicly sought someone to run in a primary against President Obama.”

Clinton added: “I’m going to defend President Obama.”

Over the last week, the liberal writer Ta-Nehisi Coates seemed to go as far as accuse Sanders of racism with the piece: “Why Precisely Is Bernie Sanders Against Reparations?”

“The Vermont senator’s political imagination is active against plutocracy,” the sub-headline of the piece stated, “but why is it so limited against white supremacy?”

Another liberal writer, Jonathan Capehart of the Washington Post, wrote a piece this week that criticized Sanders for associating with Cornel West.

“My problem with Sen. Bernie Sanders and his outreach to African Americans can be summed up in two words: Cornel West,” Capehart wrote this week.

Added Capehart: “I and plenty of other African Americans won’t soon forget the deranged ravings of the revered Ivy League professor against President Obama.”

Some of the criticism has come from inside the campaign. Last week, Tyrone Gayle, a communications aide for Clinton, re-tweeted a user accusing the Sanders campaign of stacking the stage where Sanders spoke in Birmingham, Ala. with black voters.

RISKY…. Hillary Clinton communications guy @TyroneGayle just RT’d this, implying Bernie stacked stage for optics >> pic.twitter.com/RE2CzFaqox — Dylan Byers (@DylanByers) January 20, 2016

And some of the criticism has been more subtle. According to polls, Sanders is performing best in New Hampshire, where a CNN/WMUR poll showed him up 27 points on Clinton. Senate minority leader Harry Reid has criticized the demographics of the state, saying: “You go to New Hampshire, there are not any minorities there, and nobody lives there.”

One of Clinton’s most active surrogates, her daughter Chelsea, is also weighing in on racial issues. On Friday, she tweeted her disapproval of actress Charlotte Rampling saying the recent protests about the Oscars amount to “anti-white racism.”

“Outrageous, ignorant & offensive comments from Rampling,” Chelsea Clinton wrote.

Outrageous, ignorant & offensive comments from Rampling. https://t.co/h7r1HNND5E — Chelsea Clinton (@ChelseaClinton) January 22, 2016

Speaking to the New York Times, Michael Briggs, a Sanders spokesman, reacted to Brock’s criticism of his ad. “Bernie Sanders, as everyone knows, has one of the strongest civil rights records in Congress. He doesn’t need lectures on civil rights and racial issues from David Brock.”

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