Of course I wasn’t seriously suggesting it’s a case of civil war looming. But the situation is . . . interesting. (And Obrador has hinted at calling for nationwide street demonstrations, and who knows where that may lead?) And at least tangentially relevant to this discussion.
I know - if there is a cival war in Mexico, we could all go to Mexico City and protest. That would show them!
And then they would all lay down their arms and try to sell us things! 
And it outlawed the schooling of minor children of illegal immigrants amongst some other things. It was also declared Unconstitutional.
The OAS was a clandestine organization using assasinations and terrorism against supporters of an independant Algeria. When de Gaulle eventually agreed to negotiate about Algeria’s independance, he became for them a traitor and a prime target.
We also recommend that you secure the educational needs of your children in the country where you can legally reside.
The history of prop 187 in the courts was rather unusual (it was the ruling of a single judge), and the appeals process was terminated by the then Deomcratic governor of CA. But in either case, it can hardly be cited as evidence that Americans want to shoot Mexicans crossing the border-- which is why Der Trihs brought it up in this thread. So what is your point-- do you agree with him on that assessment?
As far as a refugee situation goes, it would probably start with refugee camps at the border towns. If it got really big, then I could see a problem. But, Mexico is a big country. Depending on how the lines of control lay out, there may be space internally for people displaced by the fighting.
My first thought when I read the OP was, “Wow, we import a lot of large items from Mexico.” Both of my current vehicles, and the one we traded in last, were Hecho en Mexico. All three were from different manufactuers. I have seen lots of other consumer goods that are manufactured there. I think that the destabilization of what at least seems to be a large importer will have negative effects on our economy. Who knows, maybe the main effect in the U.S. will be to make us more dependant on China.