Cheers!
Except for Mexico City, the entire country is like the US in 1850 ie essentially empty before the West was opened. For starters, their coastline is around 15k miles!
So, imagine a re-play of the huge push to the Western states after the railroad was completed. As an offset to new Dem voters (socially conservatives, however), annexing Mexico would put the final nail in the coffin of every high-tax liberal state in the US.
It's time to stop playing defense and think offense by reprising Manifest Destiny.