
I’m not trying for inflammatory or edgy. But one of the Mexican border states would be a likely candidate for a US state. I’m not singling out one in particular, rather it’s important to point out that those border regions have been better connected into the US than they are to Mexico DF.
At one point the Yucatán attempted to become an American state. Like Texas, Yucatán had declared itself independent from Mexico. It was ruled by a mestizo ruling class, and was dealing with massive rebellions from the indigenous Maya. In desperation, Yucatán asked to join the USA in 1847. A bill passed the U.S. house but failed in the senate. The Yucatán elite would ask for British help, an eventually give up and rejoin Mexico in exchange for assistance putting down the rebellion.
Something similar happened with the Dominican Republic in 1869, which was terrified of being invaded by Haiti. It was seen as a part of the Monroe doctrine and a place for free African Americans to move too. It faced varied opposition. White racists did not like mestizo people becoming US citizens. Meanwhile progressives like Charles Sumner opposed US imperial expansion. It got a tie vote in the senate rather than the 2/3 needed to approve it.
Yesss Chuck sumner totally broke with the Grant administration over this — Grant wanted it ostensibly as a refuge for former slaves, Sumner was very critical of US imperialism. He actually broke up with his best friend(?) over this who was the founder of the Perkins Institute for the Blind (and the husband of Julia Ward Howe who wrote the Battle Hymn of the Republic)