Not so sure. With a national popular vote system, I can see Republican campaigning, for example, in Upstate New York or a Democrat campaigning in certain parts of Utah.
Sure. I didn't mean to imply that those two states would be the primary ones, but rather that anyone expecting 50-state strategies would be disappointed. Campaigns will go where the money is most effective, and proper GOTV efforts require sustained targeting. That puts limits on things.
However, I think one benefit may be that the areas which are targeted would frequently change with different candidates and different trends at the time. So there is that, but arguably not much different than how swing states come and go
(eg, Colorado/Virginia/North Carolina were not swing states until Obama). Cruz may put enormous resources into the South while Kasich would focus more up North in addition to the South. That sounds plausible, right?