According to the rulebook, the Constitution trumps any and all laws Congress makes, and any unconstitutional law is unconstitutional from the moment it hits the books.
Conducting any form of surveillance without a warrant (the constitution doesn't talk about citizens) is illegal.
However, enforcing this requires something more than a rulebook. The USSR and the PRC had elections too; like any playground bully, the response of the state when caught cheating is always "Oh yeah? You and what army?"
In Egypt, just before the collapse of the Mubarak dictatorship (and just before the rise of another, slightly friendlier, military dictatorship), the Egyptian Army rolled tanks into Cairo. But the attitude there was far from confident. So much of the country was taking part in the demonstrations. Would the soldiers fire on a crowd that might include their brothers and mothers? The answer was no, and rather than have their subordinates turn on them, the military leaders turned on Mubarak.
In the United States, what will happen when we have our own Tahrir Square? Armed with the finest of "less-lethal" weaponry, from LRADs (sci-fi sound cannons that for those not in the loop about cool new police tech) to tasers to pepper spray to tear gas to drone surveillance, and just maybe weaponized drones, will the police, faceless and uniformed, be willing to launch tear gas into a crowd containing their families?
Personally, I think the answer is yes, because the American government is simply better at doing its job than the Egyptian government, and that is why Mubarak had tanks while Obama has LRADs.
Get ready for the boot on your face, HNers. If it's not already here it'll arrive soon, and it won't go away.
The Supreme Court has ruled before that the constitution only partially applies to places where the US has jurisdiction but in which it is not a state (see the Insular Cases). Imagine a place where the US has no jurisdiction at all!
Boumediene v. Bush (2008) seems to slightly reverse this idea (people in Guantanamo Bay have constitutional rights!)
Maybe the best course of action is to invite Merkel to file suit against the NSA. There's well documented facts of warrantless wiretapping of her phone.
Article 1, Section 8 of the Constitution demonstrates Congress' authority to create an institution such as the NSA, whose main missions are (according to their website at least) Signal Intelligence and computer network protection. At least in the abstract, this isn't absurd
So much judicial precedent since the beginning of time has meant that people outside the US have less rights than those within it (I do not agree with this concept, but it is a core part of the judicial system as it is today).
Congress continuing to fund the NSA is the legal basis for their existence.
Mentioning slippery slope here is kind of funny since FISA is actually a counter-example! These things used to have no oversight at all! Now there is some minimal oversight (I stress minimal) from the judicial system.
The gov't is 3 separate bodies, each with their own way of viewing things. The judicial system has pushed back on the executive before on these issues, as has the legislative branch. Things are terrible but the system works, if barely.