I could be convinced that Walker is losing, especially given the Supreme Court/Special Election results, but he's not down 13. Nope, no way, not possible.
Collapses happen. Think of Dukakis in the 1988 Presidential election and H. Clinton in the 2016 Presidential election.
Whether Scott Walker loses by 3% or 14% will not matter in Wisconsin law. He will no longer be Governor. Walker is polarizing and confrontational, which is not good for winning more than one big for re-election in a state with a near-even partisan divide.
He is behind by a margin far out of the margin of error for most polling (about 4%). Marist may have a D bias, but not that big a bias.
