58% Of Attendees at Trump’s Wisconsin Rally Were NOT Republicans, Up from 43% in Ohio
My suspicion is this: there are a LOT of 'independent' voters in Wisconsin. They've never joined either Party, or if they did, membership lapsed years ago. So it's no big trick to find "not Republican" party members here.
That doesn't mean that Trump is not drawing from traditional (D) voters, of course. He is. They're called "Reagan Republicans," just like the Teamsters in 1980.