Former Vice President Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE’s new South Carolina ad accuses Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) of not supporting President Obama in 2012.

The ad asserts that Sanders was “seriously thinking” about competing against Obama in a Democratic primary.

The ad quotes the Vermont senator telling the Thom Hartmann Radio Program in July of 2011, “I think it would be a good idea if President Obama faced some primary opposition.”

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“Bernie Sanders was seriously thinking about challenging our first African American president in a primary,” the ad says.

“When it comes to building on President Obama’s legacy, Bernie Sanders just can’t be trusted,” the ad later says.

Sanders was asked about the ad Monday evening at a CNN town hall and vigorously denied the allegtions.

He called the idea he planned to challenge Obama “not true at all” and part of campaign “silly season.”

“I did not give any consideration to running for president of the United States until 2015,” he said.

The tough ad comes as Biden and Sanders battle for support in South Carolina, where African American voters will be a huge constituency.

Biden increasingly appears to be facing a tight race in South Carolina, where for months he had a wide lead in polls.

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Sanders has closed Biden's lead in some polls after his victories in the Nevada caucuses and the New Hampshire primary. Sanders has 45 delegates, while Biden has 15.

Biden has long touted his strength with black voters, and has underlined his work with President Obama as part of a bid to strengthen his case with African Americans.

Updated at 10:10 p.m.