I think he'd get 10% nationally, win UT, ID, and WY, and flip plenty of states to the democrats. Something like this:
Give or take MS, GA, and MO.
Hmm... WY has Mormons, but it also has a bunch of coal miners. I think Trump would narrowly hold the plurality there. Also if he picks Martinez and endorses immigration reform, I wonder if they could win parts of the SW or throw them to Trump?
Coal miners in Wyoming are different to Appalachia. In Appalachia the whole community is (was) built around the industry and so it dominates the scene. In Wyoming it's ephemeral workers with much less attachment to the job or even the area.
Romney is an longtime opponent of immigration reform. I doubt many Hispanics would find it very sincere, and it's not really the most important issue for them.
Romney's biggest issue is being seen as a candidate of the National Review/WSJ editorial board.
