Tea-Party-Backed Ted Cruz Wins Senate GOP Runoff in Texas - WSJ.com
I suspect that Dewhurst's highly negative campaign turned a lot of people against him.
As the National Review put it:
...Mr. Dewhurst’s vulgar and dishonest campaign of scorched-earth ad hominem against Mr. Cruz raises serious questions about his judgment and his commitment to conservative values. Yes, Ted Cruz for Texas - The Editors - National Review Online