my understanding is that it's not great, but it's more perception than reality.
With the exception of coal country (of which there aren't anywhere NEAR as many coal workers as the GOP would like voters to think) and US oil fields (which were in a boom that went bust when the OPEC started flooding the market and there is nothing you can do there) middle america isn't in some state of irreversible decay- but there's a PERCEPTION that it is.
http://www.politico.com/magazine/story/2016/07/rnc-convention-gop-delegates-economy-is-great-214068
you'll see over and over these people point to the decline in manufacturing jobs, and the need to bring them back as some kind of magic bullet- but there was nothing magical about manufacturing. What those voters are missing are UNION jobs with good pay and benefits that just happened to be in manufacturing. Bringing television or shoe manufacturing back from overseas so they can make minimum wage over horrible hours and conditions would do nothing. But Republicans have ironically convinced them all that Unions are the problem, so they dismantle them and vote for "right to work" laws that make it harder for high paying jobs to exist.
agree.