You won’t have to do much but sit back and watch. The food and energy for blue areas comes from red areas. Stop it all and the city populants turn on each other as starving animals would.
I think it will be pretty easy for the red states import food from all over the world.
It would take very little to plunge the blue cities into complete chaos. But if a civil war is waged, everyone will lose to some degree because the economy will suck badly and inflation will go through the roof.