Why can't Cruz be very vulnerable? He spent most of his first term ginning up a failed run for President. He shut down the government once over Obamacare, yet it is still the law of the land even though his party controls Congress and the White House. So there's a strike against him with the base. Oh, wait there's more; people like Obamacare now. A lot. So there's a strike against him with independents. Not only that, he's an unlikeable toad with weak approval ratings in his own state. There's also a child detention camp crisis brewing in Texas, thanks to a President that is unpopular in Texas. There's a border trade with Mexico crisis brewing in Texas, thanks to a President that is unpopular in Texas. And an agriculture-focused China tariff crisis brewing in Texas, thanks to.. you get the idea.
All of this stuff doesn't add up to a comfortable Cruz win. Nobody should be shocked if Beto pulls this off or the spread comes down to a few percentage points.
It's Texas.
2 months before the midterms of 1994; in the Texas gubernatorial race, Ann Richards was leading Bush 47% to 43% with 10% saying they did not know who they would vote for if the election was held today. Furthermore Bush was still considered a long shot victory then:
It would not be far-fetched for Cruz to somehow lose in the end or barely make it across in the end.
If somehow Latinos actually bothered to turnout or with all the demographic/generational changes coupled with the terrible national environment under Trump came together in a perfect mix then its possible for Cruz to loze.