Texas is now, according the US Census Quick Facts:
Population 25,674,681
45% non-Hispanic white
38% Hispanic
12% Black
4% Asian
It's generally accepted that around 1/3 of the Hispanic population is in the US illegally. That's probably higher in Texas.
But still, a significant number of Texas Hispanics have to be voting for Republicans. I think this illustrtates that Republicans do well among the established Hispanic families that have been part of the US for several generations, and not so well among new arrivals.
There could be around 3,000,000 illegals in Texas. Amnesty would probably add a million or more of current illegals to Texas' eligible voter totals. Amnesty alone could turn Texas into a battlground state by legalizing the current illegals there.