For example I came across a situation recently where someone want to export a few year old Mercedes vehicle from Canada to USA, and it was impossible as the standards were not harmonized, and the manufacturer would not certify the vehicle as meeting the USA standards.
After the Mexican-American war Polk wanted the US to expand all the way to modern Santiago de Queretaro and could have easily done it but the guy who ultimately negotiated the treaty on the US' behalf was against the idea of US expansion so negotiated against the US' own interests.
If those two decisions had gone the other way then it seems likely that all of Mexico if not all of Central America would be part of the United States today.
> Article XI. Canada acceding to this confederation, and joining in the measures of the united states, shall be admitted into, and entitled to all the advantages of this union: but no other colony shall be admitted into the same, unless such admission be agreed to by nine states.
https://www.archives.gov/milestone-documents/articles-of-con...
The power of the us + Mexico would be very similar to just the US.
From a prosperity perspective, that seven percent will grow, especially when Mexico can benefit from strong US institutions. You also have to factor in that the entire pie will get bigger due to free labor and capital flow, which will be the main win here. So it won't be 107 percent it'll be 107 percent times by a factor.
Mexico is also culturally adjacent to the US being a Catholic majority country and immigrants from Mexico have a history of integrating quickly.
Transfer payments may be needed to make the US lower classes "whole" given they're now competing with so many more people which will dilute their wages. But if done properly it should be a positive sum situation where everyone benefits.
Pan-Americanism is a fundamentally good idea if implemented gradually and carefully.
With NAFTA and USMCA the US can reap all the trade benefits without any of the liabilities
Turn on the immigration spigot and control population growth at will