It's the whole thing the left is getting all riled up about lately. Gerrymandering.
The left wants to redraw the state districts to give themselves more power. But to do so, they have to have a bunch of districts go deep into the major cities, because that's where the majority of their voting base is. It's how you end up with long thin and convoluted districts which are almost if not always blue, with red completely surrounding them.
Visuals might help...
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The thumbnail alone might give you an idea, but the main reason why states like Virginia and Illinois are blue isn't because most of their population vote democrat, it's that the districts themselves are drawn in such a way to give more power to the Democrats, by extending the dark blue city districts out into the rest of the state.
IOW, it's legal cheating.