I think the split comes down to which part of business benefits from free trade and immigration.
I heard some good analysis on the TPP, for example, which shows that as the US doesn't really export much in the way of things anymore, it's their service industries they want to support (e.g. financial services, copyrighted materials) and these are the biggest beneficiaries of the TPP. By contrast, as part of the negotiations, they're prepared to write off things like dairy (giving NZ farmers access) because that isn't such a big part of the economy and the end result will be cheaper milk for consumers. The problem for Republicans is that those sectors that benefit from TPP (Wall Street, Hollywood, Silicon Valley) support Democrats more.
It is interesting, and I feel like ideologically, the gop may have to shift towards more protectionist policies now. I'm not sure they can just take up the positions of the Wall St Journal now. Their economic position has been low tariffs, low taxes, low economic borders. But who is going to vote for that now, the wealthy? As you point out, they don't vote democrat. They basically have the manufacturing workers and agricultural workers left, but they're getting hurt by free trade. So the gop's economic demographic is shrinking, and the people who used to just vote republican are starting to lose their jobs. There are winners and losers in trade, sure, but it feels like the losers used to vote gop and are now wising up.
Trump is smart, he sniffed the wind and realised that there were more votes in Main St than Wall St. Problem is that the gop is still tied to Wall St policies. While they may not like Trump, I doubt promoting a guy who supports the TPP or immigration reform is going to help them either. That said, it'd be interesting to see how many republicans and how many Americans in general actually support the TPP and free trade in general.
The immigration one is trickier because the biggest beneficiaries of cheap, illegal immigration are agricultural producers. That's one that is a difficult line to walk for Republicans. I am guessing their pitch during primaries is to the "they took our jerbs" crowd, when it isn't illegal immigration that's done that, it's the way that the economy has restructured in recent decades.
Yeah but I'm not sure saying "don't blame Jose, blame that machine that's replaced your job" will work for many of the base. And while there are agricultural producers who benefit from cheap labor, there's workers who used to work for those producers but have been replaced by a foreign worker, or a machine.
That said, it'd be interesting to see where the dems head on issues of economics too. Do dems support free trade and immigration reform? Is there a part of the left who wishes for a return to protectionism and anti-globalisation?