Too late for reform.
Issue an ultimatum to mexico: if you keep pushing off your undesired population on the US, the US citizens want something in return.
That something will be all of the current mexican territory from the tropic of cancer north, including ALL of baja california AND the gulf of california.
The desrcibed area WILL become property of the US, and you will have no say in the matter.
If the mexican government insists on dumping a 1/4~1/3 of their population on the US, well the US will take a 1/3 of you country to recoupe our costs. If you object to this, then will just take ALL of it from you, and if you fight back, we’ll just hand your la raza asses back to you.
Anything but this. The last thing we need to do is hang that kind of millstone around our necks. That would be like inviting a polecat into our national house, so to speak.