It's the urban-rural split.
In 2008, Obama set a record: the lowest percentage of counties in the USA won by a winning presidential candidate (28%). And that was in a fairly landslidy year.
In 2012, he smashed that record: 78% of US counties voted for Mitt Romney, but Obama still won both the electoral college and the popular vote convincingly.
In 2016, Clinton lost the electoral college, but won the popular vote by several millions. She won just 17% or so of America's counties; the other 83% or so went to Trump. On the other hand, the average Clinton county had about 135k people in it, whereas the average Trump county only had about 23k.
If you look at a county-by-county map or an American election, it's invariably a see of red, with the occasional tiny spec of blue which just happen to be where most of the people live. If you look at a cartogram, with the counties scaled by population, you see something like a mangrove swamp, with swollen islands of blue separated by shrunken rivers of red of approximately equal area.
When Republicans talk about "urban" voters, I used to assume that that was just a dog whistle. I still think there's probably a large element of that, but I also think that to some extent they probably are genuinely talking about urban voters.
I think that the clearest way to define a cultural split in America is not red-state vs blue-state but rural vs urban. There are some conservative cities and some liberal or at least Democratic rural areas, but overall the cultural split is spectacular and widening.
The Democrats have problems with white voters, richer voters, male voters and less educated voters. But above all, I think they have a problem with rural voters (the GOP, by contrast, have a problem with urban voters, thankfully).
People have talked about racism, and about economic anxiety, and about anti-establishment feeling, but for my money the biggest factor in Trump's victory was that, even though he lives in a city, he managed to present himself to rural voters as one of their own, whereas Clinton came across as urban (since when was "metropolitan" a pejorative?) to her finger tips.
If you've watched the first of the Hunger Hames films, consider how the idyllic rural community at the start is portrayed and how the evil decadent city is portrayed. That really is how a lot of rural voters see it, I think.
The most important property I'd like to see in a future Democratic candidate is a background, and in particular a manner, calculated to appeal to rural voters. And I'd like to see their speech writers take a bunch of confetti with words like "rural" and "heartland" written on it, and scatter it over each of their speeches.
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I want gun control laws, too, but there are so many guns out there, I believe that if someone really wants one, they'll easily find a way to get one. Or multiple guns. 
I reminded a Trump supporting cousin that Obama & the dems never took his guns away, like he feared they would.
In fact I have posted several times, most recently in this subthread, about the pervasive proliferation of RW talk radio, its impact, and how liberals/progressives need to get on that bandwagon in a big way.
But there are certain "cultural" (or perhaps a better term might be "regional"
practices that have evolved in rural areas and in urban areas, that are distinct from one another.
For example, it is known that when growing up on dairy farms, families pick up the gut bacteria needed to more easily handle raw/unpasteurized milk, whereas those not similarly exposed end up with digestive issues and worse. And this results in a battle between the farmer and the federal government regarding milk pasteurization before mass distribution - and that causes the sparks to fly. The message then becomes - "government bad".
Another example - and one that is a tragedy going on right now - often when you live in areas where the houses are many acres (perhaps hundreds) apart, your water most likely comes out of a well. Yet in urban areas, there is some municipal water supply that pumps water to areas within the municipality after treatment (hopefully). Assuming the rural dweller doesn't have fracking nearby, the ground water is generally clean and perhaps needs some filtering for sediment and occasionally softening. However the city water needs much more given that the water is most likely coming from a (polluted) river and may be pumped through old lead-soldered pipes. So drinking water standards were put in place (which costs money), and this was done to at least try to assure urban dwellers that the water is closer in quality to those in (clean) rural areas. But standards like this were often applied everywhere (urban and rural), and the balking against that began...
You can take issue after issue (I know "guns" were mentioned here) and for some issues, there is not only a total disconnect between rural and urban, but it has devolved into shouting matches because what works one place does not work in another... IMHO one can't throw the whole discussion out because of that. There needs to be some type of adaptability.
I.e., each "region" and living circumstance has "special needs" and IMHO, there should be discussions about what those needs and circumstances are. Some might argue that this leads to a "patchwork" of regulations. Perhaps it does... but then if grouped properly and termed differently, the "patchwork regulations" become "agile (adaptable) regulations", created to tailor to several broad (but similar) categories vs "everyone".
It's like, "We will replace those jobs with green energy jobs!"
Are people supposed to believe that? I sure don't.
How's that going to come about? Are a bunch of solar panel manufacturers going to move into an area full of people with insufficient education to carry out much of that work? Are private businesses in those fields going to hire a bunch of people over age 40 to "start over" in that line of work, even if they recently completed coursework to help them do it? I doubt it, and I think they doubt it too!
Proposing FDR-like work projects with government oversight would be more believable, but I think too many neoliberals don't want to break away from their faith in free markets.
