All he needs to do is run ads featuring the actual Joe Biden in word and deed.
A snafu like this would have sunk W or Trump or a myriad of other candidates. For the left this just makes crazy ol' Joe endearing.
I remember when candidate GWB did not know the name of the head of state of one of the countries that ends in stan. No one knew it at the time but that did not stop the leftist media from bringing it up over and over as proof that W was stupid about world affairs.