Cruz was helped immensely by an inept Dewhurst campaign that blew all it's cash on negative ads.
As any Poli-Sci major will tell you, negative ads are supposed to depress turnout. But used in the massive quantities Dewhurst did it had the opposite effect, people turned out in droves to vote against him.
Now that Poli-Sci major that came up with this scheme for Dewhurst can go back to serving latte at Starbucks.
As any poli-sci major who hasn't done any research might tell, but the fact of the matter is that negative ads don't have much effect on turnout either way. See here, here and here.
One of the biggest myths about negative ads is that they depress turnout. Studies of the date suggest negative ads do not depress turnout. There's also no evidence that negative ads in large quantities lead voters to vote against the person releasing the ads.