They figured that out and are prepared.
"Robert Pastor also advises the creation of a North American Parliamentary Group to make sure the U.S. Congress does not impede progress in the envisioned North American Union. He has also called for the creation of a North American Customs and Immigration Service which would have authority over U.S. Immigration and Customs Enforcement (ICE) within the Department of Homeland Security."
Americans do NOT like to be part of a collective/UNION.
They would be willing to annex Canada if they drop French and could annex Mexico if they don't mind killing off all their drug lords.