The state has always looked as if it would be one of Mr. Trump’s worst. This was true even before the primaries began. Polling by Civis Analytics and Marquette Law both showed Mr. Trump faring much worse there than nationally, just as was the case in neighboring Iowa, where he wound up losing to Ted Cruz.

The results of other primaries have consistently pointed toward Mr. Trump’s weakness in Wisconsin. We’ve frequently published articles using models to predict the results of contests based on demographics, and both models — one after the Super Tuesday primaries and one after the March 15 primaries — projected Wisconsin to be one of Mr. Trump’s worst remaining states.

Our estimate remains virtually unchanged from what it was after the Arizona and Utah contests March 22: a five-point edge in Wisconsin for Mr. Cruz, almost exactly aligned with the polls. The model does not account for the possible redistribution of Marco Rubio’s former support.

Why is Wisconsin such a problematic state for Mr. Trump? It’s not the worst possible state for him (Utah was), but it does force him to confront some major weaknesses.