Inside the Bernie Sanders campaign, there was a strong belief that sharp attacks against Joe Biden for previously embracing cuts to Social Security were what propelled him to victory in the Iowa caucus.

Now, as Sanders finds himself slipping behind Biden with the nomination process roughly halfway over, he is going back to that line of attack as he tries to overtake the former vice president once more.

During Sunday night’s debate, the two candidates had a sharp back and forth over the topic of Social Security, with Sander once more accusing Biden of embracing entitlement cuts and challenging those who didn’t believe him to check out YouTube.

"My Lord, Bernie, you're running ads saying I'm opposed to social security that Politifact says is a flat lie," Biden said.

Sanders then challenged Biden over whether while he was in the Senate, Biden had talked about the "necessity, with pride, about cutting social security, cutting Medicare, cutting veterans programs?"

Biden said no, leading Sanders to call on viewers to "go to the Youtube" and check for themselves.

Biden defended his approach, saying he has shown how he "will increase social security," but that wasn't enough for Sanders.

"One minute you said, I was not on the floor. The next minute you say, well yes, there was a reason why I was worried about the deficit,” Sanders said. “Maybe that's good reason, maybe it's not. All that I am saying is you were prepared to cut and advocated for the cuts."

Biden, growing frustrated, said he never voted to cut social security.

"I voted to protect it," Biden said.

The record with respect to Sanders is pretty clear. He’s steadfastly pushed to expand Social Security. Biden’s record is trickier than that, which has opened him up to lines of attacks. As a Senator he supported a balanced budget amendment, that advocated feared would harm the program. He is on video talking about his comfort with putting entitlement programs on the chopping block. But he also called for expanding Social Security too.

The debate, in the end, is really one of semantics. As Vice President, Biden was part of negotiations with House Republican leadership over how to avert the debt ceiling being breached. And as part of those talks, Biden and Obama did entertain Social Security cuts. Sanders has said that is tantamount to support. Biden has insisted he was merely willing to swallow it as part of a deal. Where one comes down on that debate could affect the course of the remaining primary.