You already became Mexico. When? When your politicians decided a vote is more important than the country. I.E. They pass bills thinking about how many votes that bill is going to give them, not if that bill is going to make the USA better. Both Republicans and Democrats.
So, politics like that only benefit politicians, not the rest of the country. Are you becoming Mexico because of the mexicans? Maybe, I don't know. But with that political system you will (very, ver, very slowly) become a third world country. And the only ones to blame will be your politicians (not Mexicans).