That's a very broad brush. For instance, coal mining is not relevant at all in Michigan and not very big in Ohio as well--in terms of the Mid-West, it is mostly in Pennsylvania, West Virginia, Kentucky, and Indiana.
The auto industry is more relevant to Michigan and Ohio, for instance, and Obama won many of these voters with a strong anti-NAFTA and pro-auto bailout message. The Republican Party for the most part hasn't been hitting the same notes--I believe every other Republican candidate except Cruz supported the TPP.
Even the American auto industry was opposed to the TPP:
http://www.nytimes.com/2015/10/06/b...-from-the-trans-pacific-partnership-pact.html
The same applies to other manufacturing jobs. Forget about the coal industry--or agriculture and construction (where most "illegals" work). I think the margins are small enough in Michigan (not sure about Ohio), that even just focusing on this narrow group of previously reliable Democrats can be enough to tip the balance.
There are larger numbers of anti-trade, pro-manufacturing Democrats in Congress than there are similar Republicans. Just look at how the votes on TPP fast-track authority went--only 4 GOP senators voted against fast-track, while 33 Dems voted against fast-track. Only 13 Dems voted in favor of fast-track, while 47 GOP senators voted in favor. Similar numbers in the House. These platforms are still part of the Democratic base, even if a large number of these voters went for Trump in the last election.
Finally, because young and college-educated people are leaving the Rust Belt every year, while the electoral vote apportionment won't take place until after the next election in 2020, there will be proportionately more of these voters in the Rust Belt in 2020, while young and college-educated will be fewer. You will not win a Rust Belt state in 2020 if you cede these voters to the GOP.