And as far as illegals being covered under the constitution, Chad covered it all but I also want to point out that besides rights to due process and what not, checking for ID at a protest is about more than just that.

You can not assume that everyone engaged in a protest rally is an illegal. There are all manner of groups involved in these protests, from Hispanic organizations to the Roman Catholic Church, unions, and on and on and on. There are immigrants rights groups covering all manner of ethnicities as well - Hispanics are the most prominent, but Asian rights groups, African, and even European are all involved (somewhere around 5-6 percent of all illegal immigrants in the United States are European or Canadian). There are certainly a very substantial number of legal immigrants, citizens, and residents also involved in the protests.

So if you are a citizen and choose to involve yourself in this protest, for example, and that puts you at risk of being searched, seized, stopped and questioned by federal law enforcement officers, I'd imagine you have a very good civil rights/constitutional rights violation case against the gov't on your hands.

You were exercising your right to freedom of speech and assembly and for that reason alone, officers questioned you. Regardless of whether they stopped and questioned 1,000 actual illegal immigrants before reaching you, the first citizen, it was still unfair for them to assume that you were in the country illegally simply for having involved yourself in this protest and rightfully assembling and exercising your free speech. Their search and questioning of you only occurred because you were exercising your constitutional rights.

And that would be wrong. Very wrong.

What next? Stopping and questioning everyone at an anti-war rally under suspicion they are terrorists or terrorist sympathizers? Questioning everyone at an anti-tax rally under the suspicion they've all cheated on their taxes? If you're at an anti-death penalty rally can the government assume you've committed a capital crime at some point and are only out to save your own ass in case you're busted for it?

You might say I'm using a slippery slope argument here but really, our constitution was written and the judiciary enforces it with the slippery slope in mind.

If the government is going to act against you for exercising your constitutional rights, they've just violated them.