I used to think Santorum would stay in no matter what. I'm now seeing him dropping out 4/24.
If the polling gets worse, he may drop out sooner so he can claim his upcoming loss was due to the nomination being decided before Pennsylvania voted. Which in many ways is true. I think Santorum lost Wisconsin because many Republicans decided that it was more important to end the race than it was to find the best candidate. Santorum's losing the support of those who were hoping for a brokered convention that would choose someone who isn't running as it becomes apparent that the brokered convention won't happen.