Former Vice President Joe Biden has released a new digital ad in South Carolina slamming Sen. Bernie Sanders, accusing his opponent of attempting to undercut President Barack Obama’s reelection campaign in 2012.

The video begins with a montage of anchors announcing Obama’s reelection campaign before a narrator says, “When we rallied together to defend our president and all the progress he made, they had his back,” showing footage of supporters. A photo of Biden appears as the narrator reads, “He had his back,” and more footage of supporters is shown as the narrator continues, “And you had his back.” As a photo of Sanders appears, the narrator says, “But back in Washington there was one guy with another plan.”

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The ad comes as Biden tries to use his own ties to Obama – the most popular figure in the Democratic Party – to his advantage and contrast them with Sanders, whose momentum he is trying to stop after a decisive victory Saturday in Nevada.

The ad includes audio from a 2011 radio interview with the senator from Vermont in which he said, “I think it would be a good idea if, uh, President Obama faced some primary opposition.”

“Bernie Sanders was seriously thinking about challenging our first African American president in a primary,” the narrator continues. “The Atlantic reports that Bernie Sanders told fellow senators he’d take on Obama. And Obama’s team was ‘absolutely panicked.’ Obama’s campaign manager knew it would be a dangerous threat since ‘every president who has gotten a real primary has lost a general [election].’ “

The video ends with the narrator saying: “When it comes to building on President Obama’s legacy, Bernie Sanders just can’t be trusted.”

Asked for comment, Ari Rabin-Havt, Sanders’ deputy campaign director, said: “This never happened. Bernie Sanders never considered a primary challenge to Obama. Bernie was running for reelection in 2012 and that’s what he was focused on.”

Biden has started to use this line of attack more on the campaign trail, swiping at Sanders without using his name at his Nevada caucus party, claiming he was proud to run with Obama and adding, “I’ll tell you what – I promise you I wasn’t talking about running a Democratic primary against him in 2012 like some people. I was working my heart out so he could win.” Biden’s campaign is now putting money behind that message.

The one-minute ad, called “Can’t Be Trusted,” is part of a broader $600,000 digital investment in South Carolina by the Biden campaign. It will play on YouTube, Facebook and Instagram.

Sanders, for his part, claims he and Obama are in fairly regular contact and calls him a friend. Calling Obama an “icon” in the Democratic Party, Sanders also noted in an interview with CNN’s Ryan Nobles the former president’s importance in the 2020 election. “No question about it, we need him. And if I win, I’m sure he’ll be there at my side,” he said.

Although Obama has not endorsed any candidate in the Democratic primary, Biden has tied himself repeatedly to the former president on the trail since launching his campaign, often telling the story of when he was asked to be Obama’s running mate and citing experts claiming they were the “best” president and vice president duo in modern history.

“I was really proud to be Barack’s vice president – President Obama’s vice president,” he said to loud cheers recently at a rally in Las Vegas.

CNN’s Annie Grayer contributed to this report.