A few things are going on here. First, there is a midterm effect, potentially dragging down Republicans, but that's not consistent across the country. Second, Ted Cruz is unpopular, because he's Ted Cruz. Third, money is an overblown phenomenon, but it can help challengers, and O'Rourke has a lot. Here's the thing about money, though. There's a limit to what it can do, and it is subject to heavily diminished marginal returns. O'Rourke's money hit that point long ago. One of the things that I like to do when I teach congressional elections is to graph challenger spending against challenger vote shares. It is a simple way to demonstrate an important principle to students who don't have a statistics background. As challenger spending goes up, their vote shares go up, and asymptotically approach 50%, with just a scant few crossing the 50% threshold. If it were a curve-of-best-fit, though, it would look bounded above by the 50% mark. In other words, money can get you close, but never over the finishing line. Why not? The short version is that money can overcome reluctance to vote for someone you've never heard of, but not the reluctance to vote for someone of the wrong party. Texas is a Republican state. Money helps challengers, but it doesn't determine outcomes.
I do this exercise with House elections, and the Senate is a little different from the House, but basic principles hold. Texas is expensive because there are so many media markets, but the principle of diminishing marginal returns holds everywhere. So, yes, O'Rourke is spending heavily in an expensive state, and he has closed the gap noticeably.
He's still running as a Democrat. In Texas. Against a Republican.
So, why are people paying attention to this one and not Dean Heller's race? Heller is only up by 1.7. You're paying attention to the wrong things.
*Special grudging exemption for musicians.