It wasn’t just the Dem “crossover” stunt that hurt Santorum. He helped to sink his own ship with some of the stupid comments he’s made recently. Plus, more voters have become aware of his mixed voting record while in office (support for pro-union measures, etc).
Add all of the above to the negative ads Mitt, Inc. has been running against him, and the result is toast.
Which also illustrates WHY the move to Santorum by so many, was absolutely ignorant. His record was there for all to see, before they made the choice. There are no legitimate excuses for how things have turned out.