I was born and raised on the Gadsden Purchase and most of my immediate family is either living or buried there, 60 miles north of the Mexican Maybe-Border. I don't really want to have to get a Mexican Visa just to go see my Mom's grave. Or my children having to, either. And I mean that.
Just to make the Reconquista's head's spin, we should sue Mexico in the International Court for breach of the purchase agreement because they a) now have so many of their citizens living on the land we paid them good money for, which is clearly not included in the contract, and b) because they have politicians in Mexico openly trying to abrogate the deal.
If they refuse to participate in the case it would be a good pretext to declare war on them and take some more land. Arizona really does need a nice seacoast town on the Sea of Cortez. We'll take Puerto Penasco, but all the locals have to leave....or else BOOM! He he...just kiddin'...well, maybe not that much... :^)
As I remember it (from reading--it was before my time), the Senate reduced the area acquired by the Gadsden purchase in order to reduce the amount of new territory that might someday become slave states.