They don't have standing to sue. Not unless and until the election is over and Cruz is President,
There’s not much chance of finding out. Cruz is unlikely to be the nom, and far less likely to be the veep pick. If by a miracle he became the nom, the Dem would win in a landslide.
Otoh, whether a plaintiff has standing is largely a function of the individual judge. After Grayson has shopped for the most favorable venue, don’t be shocked at the outcome. It’s a different world for liberals and conservatives.