Think about this for a second; if, under your scenario, the US was divided into two separate countries, one being red and the other blue, what's to prevent the blue countries from forming alliances with China and/or Russia? And lets say as part of that alliance, the blue states allow a military presence from either/or both countries with the possibility of nuclear weapons being placed on what was formerly American soil. And because the former US is no longer perceived as a big threat that It once was, Mexico, with the help of China decides to "take back Texas"? And then New Mexico and
Arizona, etc. It's much more complex then what we can say, but the point is with the fifty states united, there is strength in numbers, with the states broken up all bets are off.
That's why I'd be opposed to breaking up the union due to political differences; we need to be careful of what we wish for since we might actually get it.
What makes you think that the blue states and Democrats in general are NOT in alliance right now, with China, Iran, Cuba, Mexico, and other enemies of the American people ?
I highly doubt that China or Russia would approve of nuclear weapons being placed in UBSA + we are already under attack from China with biological weaponry (the Covid virus)
The strength in numbers theory is ALWAYS stated whenever a multicultural entity wrongly composed a nation tries to free itself into a true nation with distinct culture.
Mexico need not take back any land from the US. Mexico has already conquered the US, and has been colonizing us for decades with 21st century imperialism (remittances$$$$) and has been raiding our economy for $40 Billion/year (the Vikings would be envious).
Since the Democrats only allow this to secure VOTES from Mexican migrants once the red/blue slit occured, Democrats would no longer have any need for the aliens, and they would close the borders of New Mexico and California.
Like wise the red states of Texas and Arizona would close their borders too. As for Mexico taking military action against UBSA and/or URSA, not in a million years.
Both the URSA and the UBSA are heavily armed with lots of military forts, navy, air force, and nuclear arsenals. There would not be much loss of military strength.