But if Mexico (and Canada to some extent) were to become part of the united states, the Democrats, at the behest of the "watermelons" will make them stop Oil production, Mining and Timber operations.
Then we'd be completely 'up the creek and no paddles in sight' rather than just mostly 'up the creek'.
While the United States would probably turn more toward liberalism (especially if Mexico's lands are given statehood before full development; Mexicans are generally very socially conservative, and probably would become economically if they develop), the country could gain a lot, too.
Not to mention the continued erosion of US court protection for American businesses. Mexican and Canadian "ministers" can out vote us, and even if it is against our National inerest will we be bound by their decision.