Trump "screwed" up on immigration by 1) Insulting Mexicans 2) the kids being separated from their parents.
He screwed up in some respects, but the issue isn't big enough to be decisive. Obama passed a bad healthcare bill that looked even worse when it was passed and had the potential to impact hundreds of millions of people. Bush led us into war on a basis that turned out to be false, and to compound the problem, he executed the war terribly.
Trump insulted Mexicans, but he has been doing that since 2015. I'm not saying it helped, but if he had handled the immigration issue the same way but acted like a serious adult about it, I don't think it would have hurt him very much, if at all and likely wouldn't have tarnished the entire Republican brand.
He did pull us from the Paris Agreement. That flew in the face of the Left. But engaging North Korea should be seen as a good thing. The tax cuts should be seen as a good thing. It definitely put money in everybody's pockets. However, there is the national debt to consider. How many people are truly concerned about that?
The Paris Agreement pissed off the Left, but I don't believe it motivated voters who weren't already hostile to any Republican. Furthermore, he did that early on. I don't think it was on suburban moms' minds in Nov. 2018. And nobody cares about the national debt. They do superficially, but nobody votes on that.
What am I missing other than his tweets? Kavanaugh? He got blind-sided on that one. It's no surprise he'd pick a Conservative. He stuck with Kavanaugh so that was not so much policy as politics. Maybe that offended some moderates who as I said like the feeling of being "good."
I actually think Kavanaugh, if anything, helped, because of how terribly the Democrats handled it and because it amped up the GOP base.
To me, the Tweets (as well as the stupid statements, picking juvenile cultural battles for no reason) hurt him more than anything. If he had got into office, pushed the same policy agenda, but basically acted like a regular guy, I think we'd still control the House.