Trump wins where racial resentment is high. Cruz wins where it's not.
In other words, Trump wins where there is a sizeable-enough minority community where working class whites feel as if their priveleged spot in the established social hierarchy is threatened: the South, the Rust Belt north, and the Southwest. Cruz wins in places that are so overwhelmingly white that there is no such threat: the Corn Belt. (Some exception applies for candidates overperforming in their home states. )
Wisconsin has some Rust belt cities (e.g. Milwaukee), but for the most part it is a very white and rural state. The demographics don't favor Trump. A win there will end the nominating contest. A loss will drag it out for months. The latter is more likely.
For Trump it will probably be a bad week, but the following week he is going to go on a tear in the Northeast. There is no doubt he will enter the convention with more delegates than Cruz and Kasich combined, but it's unclear whether he will have the 1237 needed on the first ballot.
If he doesn't, it's believed that Trump will start bleeding delegates who are not loyal to him and bound for the first ballot. And that's where the media narrative is right now as it's the only path forward to the anti-Trump camp.