Something I keep coming back to is party bases. Democrats and Republicans alike have some core of voters who simply identify with the brand (like Ford or Chevrolet people). It's hard to say what could move them, but it has happened; yellow dog Democrats did become Republicans all across the South.
Democrats have pandered to and basically bought a large low or no income bloc. Liberalism (which the Democrats now claim) attracts a huge chunk of young voters and still others who see capitalism as unfair. Those alone count for a big base without even needing to court other blocs - religion, LGBT, by ethnicity,etc.
The Republicans have tried to compensate by cobbling together blocs like conservative Christians, but when you get right down to it the Republicans have almost nothing to directly offer most of us. It's harder to visualize lower taxation vs a check and benefits for nothing. One is more dollars per check - probably no great percentage, and the other is free stuff right there in your hand.
The only way that middle income people really benefit is in job availability, lower taxation, and a fair marketplace for goods and services; and the Republicans have been looking the other way. Example: the banking/investment meltdown(s) because Congress and regulators pandered to the banks and to real estate - they didn't protect the constituents. They've continually allowed mergers in many fields which has prevented the opportunity for better pricing. The healthcare world is a prime mess - Republicans (and Democrats) pandered to drug, medical supply, insurance companies and to the trial lawyers. Rather than address any of those (even Medicare/Medicaid drug price negotiation) they allowed the Obamacare abortion to pass - might have had the votes if they'd worked for their constituents over the years.
What the Republicans have to face is they just don't have a lot to offer unless they make some major strategy changes. Illegal immigration probably gains more votes than it alienates. The conservative Christian thing may very well be a loser - just too many young liberals - same with welfare reform. That probably doesn't leave a lot but issues that most voters can like - clean the place up - campaign finance reform, lobbying reform, anti-monopolistic policy, banking/investment reform, healthcare cost reform, foreign trade/jobs (protectionism - we'll take care of our own, thank you), cleaning up the legislative process (like no hidden agendas - one item per bill), fiscal responsibility. That means parting ties with the very select and very wealthy, and I doubt that either party is willing to go there. Beyond that and the inherent un-likability of Hillary and Bernie's "socialism", Republicans pretty much have nothing.