They can certainly attempt to mobilize a voting majority in their fantasy new country, but we've already had a pretty good precedent set on what happens when there's a declaration of secession.
There will never be an Aztlan.
Secession, no. As I noted, and as they admit, they aren't in the position to acheive their goals by violence, as many palestinians feel they are.
Some others, a minority, feel the destruction of Israel will be best brought about by flooding the country with Arab immigrants, the right of return, which won't happen either.
As I noted before, there's a body of thought in the LaRaza movement that voting majorities can accomplish a great deal. No, they won't secede. And the federal government still controls the borders. But local and state government controls the educational system. The police, including input in the level of cooperation with the feds should we ever get serious about illegal immigratin. Language, the nation doesn't have one last time I looked. A state could adopt spanish. Drivers license, voting, all local control. Welfare, a great deal of local input, and a state can adopt any program they care to, and finance it with state taxes. As they can with health care, minimum wage, and working conditions.
There's a reason LaRaza, and others, want citizenship for illegals. You're being naive if you think the only risk is secession, which isn't a risk at all.