“I helped lead the fight against [NAFTA]. He voted for NAFTA,” Sanders said in a Monday interview with CNN. “I helped lead the fight against [trade relations] with China. He voted for it. I strongly opposed the Trans-Pacific Partnership. He supported it. I voted against the war in Iraq. He voted for it.”

While political analysts caution that polling the race this far ahead of the first primaries and caucuses doesn’t accurately predict who will win the nomination, Biden’s bounce has impressed.

“I didn’t expect Biden to have a surge and he did. I think that’s a good thing for him,” Howard Dean, the former head of the Democratic National Committee and a presidential contender himself in 2004, told Yahoo News. Dean noted that when he ran in the Democratic primary in 2004, he was virtually unknown to voters at this stage in the race. Then, suddenly, “I was leading the pack.”

“I do think Biden’s move is significant, but we have so far to go,” Dean said.