In most states, many or most of the Republican delegates are allocated by congressional district, so that means urban, liberal districts with relatively few Republicans get as many delegates as rural districts with strong Republican majorities. "It’s the disproportionate influence of urban voters in the Republican nomination process that makes it difficult for Sen. Ted Cruz to make serious headway against Trump," Kraushaar writes.
Trump has been doing well in liberal areas partly because his "mainly-white supporters are disproportionately concentrated near areas with many minorities, suggesting that strained race relations may have played a role in their backing of Trump," Kraushaar reports. "He has won over white Republican voters in rural Southern counties where African Americans make up a majority of the vote, and he has performed well in urban neighborhoods where the racial composition of surrounding areas has changed over the years." Also, "Some of the urban congressional districts are gerrymandered in order to take a small slice of Republican suburban territory along with heavily Democratic urban turf."
Trump had an early lead in Wisconsin but now trails Texas Sen. Ted Cruz by about 10 points.