I've always thought that a combo of Castro running for Senate (either, but Joaquin is the stronger candidate) & Beto also on the ballot- Texas really does become dead even. Castro will naturally increase Latino turnout to some degree. And Trump & Cornyn have much lower approval ratings than Cruz (which is shocking for most ppl outside of Texas).
So if you start at Beto's 3% loss. Add a bump in Latino turnout, factor in Trump & Cornyn being less popular than Cruz (& Abbott who was also on the ballot in 2018)- and add a small bump if Beto in the nom/ for being the home state candidate...
This could be the year Texas finally falls. The Texas quinnipiac poll from a couple of weeks ago had Beto:46 ... Trump:47 (and I would imagine Castro vs Cornyn is pretty much the same).