What if, after the occupation of its capital by the U.S. in late 1847, Mexico decides to fight on in the form of supporting an anti-U.S. insurgency throughout Mexico (as opposed to negotiating the Treaty of Guadalupe Hidalgo with the U.S.)?
How damaging would such a Mexican insurgency have been to the U.S. forces stationed there?
Also, how would this have affected the 1848 U.S. elections?
Finally, if the Whigs still win in 1848 in this TL, do they offer to negotiate a better treaty for Mexico or do they--as U.S. President James K. Polk--insist that Mexico sell New Mexico and Alta California to the U.S.?
Any thoughts on all of this?