Likely R, but:
Cruz seems like exactly the kind of guy that would have a massive scandal revealed about him.
Nah, not really. He's been in the national spotlight for the better part of 5 years and has endured a presidential campaign, if there was something especially damning about him it would have come out by now.
Doesn't change the fact that 2018 is going to be a strong year for Dems, Beto is a good recruit and Cruz is now out-of-step with his state on a lot of issues. Likely R, with a downgrade to Lean R very possible in the next couple months.