Democrats are catering more toward the progressive urban voter and abandoning their core blue-collar voters. Hillary championed that she was going to put a lot of coal miners out of work. That doesn't play well amongst coal miners. If you look at the top issues their candidates are talking about, they don't really come into line with the typical blue-collar voters.
The big democrat talkers run around claiming it's the republicans that keep blocking gun control legislation, when in fact it's actually the voters who keep doing it. Following the Aurora theater shooing, the state legislature there passed sweeping gun control legislation. Several of them ended up getting kicked out of office for doing that. Not losing their re-election bid, but getting recalled and booted. Not by a republican voting block, but by the very people who used to vote democrat.
Many blue collar workers are feeling abandoned by their party because they don't really align themselves with where the party is going. Trump is able to capitalize on this by bringing industry back to America and providing them with jobs. They can either listen to democrats talk about getting them jobs, or they can see republicans getting them jobs. Which side to you think they'll vote for? (p.s. this is also starting to happen amongst black voters as well)