This is some important scholarship on how we form "our" political opinions. It starts out explaining why Republicans have seemingly changed some of their long held beliefs in the face of Trump. Another example they don't use is when Obama came out for marriage equality and approval exploded among (African American) Democrats.
Basically, while we may have some innate beliefs or tendencies, when it comes to policy, we typically just assume what our party believes, once we understand that this is what our party believes. So the GOP can go from hating Russia in 2012 to loving Putin in 2016, just because the party elites say to do so.
This is why measuring Trump's approval rating amongst Republicans tells us nothing. They love what he's doing because they are loyal Republicans not because they have made a rational decision about his "policies" whatever those may be. (There is at least some evidence that there a fewer Republicans now. Meanwhile, this poll has more Republicans than Independents in their sample, which strikes me as problematic, even though Trump is doing poorly in it.)
The good news is that if the GOP decides to move to the center, most of their voters will move to the center. However, the primary system does seem to expose that Republicans don't get their views from party elites, but rather partisan media outlets. The gatekeeper of GOP political opinion is not Mitch McConnell but Tucker Carlson.
It has been noted that the GOP actually has no ideas (and no party platform) beyond love of Dear Leader. The GOP exist primarily to "own the libs." Therefore Trump is the perfect representative of an idea-less party that exists simply to fight a culture war with liberals.
It also stands to reason that once you identify with a party that there is very little policy outreach that will cause partisans to switch sides. We are seeing some of this, also, with Sandernistas whose political identity is wrapped up in hating Democrats almost as much or more than hating Republicans. Biden adopting aspects of Sanders' program is unimportant to the hardcore Rose Twitterati.
Finally, it means that everyone fretting about the guys from the Lincoln Project coming into the Democratic tent and turning it into the party of George HW Bush have it backwards. They are far more likely to embrace climate change policies, progressive taxation and greater access to healthcare precisely because they are inside the tent.
Turns out the party DOES decide, just not in ways we anticipated.