I don't know anything about how their Governments are set up. What is in their Constitutions and so on. The only thing that I do know is that their respective Economies are not as good as the US's and this "Bringing down the playing field" that others are talking about is a bad Idea for our econmy. If this Union exsists, it could develop into a few different ways:
1. A Continental Country. The United States was originally that a group of 13 states (Colonial Nation States) united for a common goal. It was not a Country as people think of the United States today. This NAU could be a Country of 50 former US states, 11 former Canadian states, and 31 former Mexican States, or a Country called the North American Union that is 91 States big. If you have a new country, you have a new Constitution for this new country. Therefore the old Constitution is out the door as are rights guarenteed by it. Do you really want to have the people in political power to decide your new rights?
2. A Defensive Union like NATO. Then why bother? If we already have NATO, It's not broke, then why fix it?
3. A Union of Countries. If the Mexican, United States, and Canadian Governments are still in charge, then you would have Local Governments taxing you, State Governments taxing you, the Federal Government Taxing you, and now the Union Government taxing you! Even if they were to cover it up with each respective Federal Government gives it's "Member Fee" like with the UN, the Federal Government has to raise the Money somehow, right? Ultimately it comes from us. And I believe that the Union would still develop its own Constitution, so which Constitution would out weigh the other? You have a State Constitution, but do you know what is on it? You have a "City Charter" or something like it, but do you know what is on it? But you do know what the US Constitution says. That is the document that this Nation is built on, if we have another Constitution above the US Constituion, which one do we follow?
I would prefer to keep the Countries as it is now, it aint broke, why fix it?
Do you really think that the sovereign states of Mexico and Canada would like to lose power to the US?