I'd have to look, but I believe it's Article IV of the constitution that says the feds will ensure each state a republic form or government and protect each state from invasion.

What Arizona has on its hands is an invasion. Illegals pour across the border. They come to the US for the government programs. Can't blame them, but it's hard on the economy.

But if the feds would hold up their part of the statehood contract, the border wouldn't be porous. Aren't we "at war with terrorism?" The first thing you do in a war is shut the borders to protect attack. So why is the Mexican border so open? Arizona had to do something to protect itself.

It'll be interesting to see what the courts say.