The former top political adviser to Barack Obama, David Axelrod, has likened the Bernie Sanders surge to Howard Dean's in the 2004 election. Axelrod made the comments on Twitter.

"Bernie's huge crowds recall Howard Dean's in '04 when he rallied 'Democratic wing of Democratic Party,'" said Axelrod. "Dean was impactful, but Kerry won."

Dean's campaign manager, however, appears to think the comparison isn't completely valid.