ok, (I know) you can always argue either side of an issue successfully (even if you don't believe in it) based on two things - one, that you can clearly define the point you are arguing - so here you don't want to be guessing that the above is your point, you want to be certain of it - (and I am not clear on that - is your argument specifically about "how they justified making it legal?")
and then secondly, keep your own opinion separate from it - as it just makes it that much easier to argue-
then next, I'd start with the new law on it -
http://www.epic.org/privacy/terrorism/usapatriot/default.html
and build from there - (and I think there was a renewal or update on the Act recently, which may have made changes to it, so you want to check that as well) and address how and why it supercedes/circumvents what was law in place prior to the ACT.
and I would also research (as Lisa suggested) all arguments that already support this position, eg., (US Supreme Court)
and lastly, but most importantly, I would then go looking for then holes in my own argument by going to the ACLU website and see what they say against it - and make certain that I have covered those points in my argument -
http://www.aclu.org//safefree/resources/17343res20031114.html
one other point that may be a consideration too - does the US Constitution apply to those citizens from other counties? (although in Rumsfeld v. Padilla her was a US citizen, I believe)-
and here is a law link that has some case information that may be helpful to you as well
http://news.findlaw.com/legalnews/us/terrorism/cases/
Good Luck Megan!
PS if all else fails to win a point that your arguing, just say "well, currently it's law"!! ;)