An incumbent up only 41-35 over a barely-known challenger at this stage? Vulnerable. Cruz needs at least a 43% share of the likely vote already locked up if he is to have a better-than-50% chance of winning re-election.
See Nate Silver's Myth of 50%
https://fivethirtyeight.com/features/myth-of-incumbent-50-rule/Sure, his model was wrong about Strickland vs. Kasich -- but by his model, Cruz is in deep trouble even if he has a bare early lead.
It applies just the same to incumbent Senators as to incumbent Governors... and it is easy to say, but Cruz is in Texas. Sure -- and Feingold was a much-respected Senator from Wisconsin, a state famous for progressive tendencies. Governor Scott Walker and Senator Ron Johnson have smashed that reputation!
Demographics are trending against Republicans in Texas. The Mexican-American component of the electorate is growing fast, and even if it is culturally conservative it has its limits on political conservatism when such implies cruelty. Add to this, Texas is much closer to the American average in levels of formal education, in part because of well-educated people moving to Texas' giant cities Dallas, Austin, San Antonio, and Houston. Well-educated people have been slower to go Republican in Texas than much of the rest of the country... but the 2016 suggests that such people are catching up in Texas (see also Arizona and Georgia).
