Over the past 30 years, Republicans and Democrats increasingly differentiate themselves based on where they lie on the conservative-liberal scale. Not much room for overlap (as there apparently used to be back in the day). From a positioning/marketing/branding point of view, that’s brilliant – your Party attracts a more vocal, more loyal voter base. The problem? How do you differentiate even more? By becoming even more conservative or even more liberal, grabbing an even more vocal, more loyal, and more stubborn voter base (who also tend to be the ones contributing $$ to campaigns). Everyone else tends just vote as they used to, apathetically, despite the nuances of their political lives.
Is this process reversible?
It might take an off-shoot group of politicians that can clearly and convincingly find a distinct position somewhere in the middle (which is a pretty noisy space). Or even flip the Party-on-a-conservative/liberal-spectrum script entirely. Political difference may exist as operating under assumptions not necessarily driven by how conservative or liberal you are. Those may just be the effects of the actual cause. What that cause is I have no clue. May be linked to historic regional affiliations (“Texas Forever” etc).
Inspired by Ezra Klein’s piece in Vox.
ammar mian 
The brand has to produce results. The Republicans brand identified with their core market, but they did not appeal to other markets such as a growing non-white population, thus they lost the Presidential election.