Yes I don't understand how it will decrease the power of rural areas. If anything it will increase the power of ignored voters. Finally, the Democrats would have to pay attention to rural voters in safe States of both colours; likewise with the Republicans in urban areas.
Under a national vote, every single person has an equal influence on the vote; so campaigns might as well start buying up ad space in Nebraska or Upstate NY or Alaska; and indeed awaken moribund parties in one party states. It would also give the citizens in the territories a voice.
True