By physically being 50.1% or more of the population in some towns and voting for it. You've seen what happens in la and san fransisco when people vote for people that don't have allegiance to the country, now imagine that with Mexicans in Texas. I'm sure there already are some towns where the majority isn't American. And even if it's not it's close, and stopping that at this point would be difficult to say the least. Deporting millions of illegals is logistically difficult. Triply so if large parts of the government in those places are made up of Mexicans.
Look at London for an example. At last census count only 35 or so % of London was native born. The "Cockney" ethnic group no longer exists.