So Santorum’s criticism is that Ted Cruz...wants to actually obey the Constitution?
In a sense what Santorum is saying is true. We argue for limited government; proper understanding of federalism through the lens of the 10th Amendment etc., as the best method for advancing socially conservative principles. So when you step back and look at it for a moment, the Constitution is nothing but a tool. It is a means. So adherence to the Constitution and adherence to socially conservative principles should not be confused as being the same thing.
If one reviews Cruz's latest victories ad what is he saying? I used the tool of the federal judiciary to advance x, y and z. It takes more than saying let the states decide to be socially conservative.