Who's challenging Mitt' eligibility? No one here, he was born in the US of parents who were citizens.
You realize that by citing the 1790 act, it shows that prior to the act, when the Constitution was written, such persons born of citizens outside US jurisdiction were not considered natural born? Else why bother with the law? Since the Constitution came first, the definitions when it was written are what count, unless it has been amended to change them.
As long as Santorum's parents were citizens when he was born, and he was born in the US (and possible outside it if one of them was in the service of the country serving outside of it) then he's good to go. \
Santorum’s father immigrated to the US when he was seven. It is pretty likely he was a US citizen when Rick was born. The pathway to citizenship was much simpler then and welcomed. As I understand it, his mother was as US citizen at birth.