The district court, in a three-page analysis — mostly consisting of block quotes from opinions in the Supreme Court's United States v. United States District Court (Keith) case — concludes that the program is "obviously in violation of the Fourth Amendment." The opinion, however, doesn't even mention the arguments that

1. the Court has expressly held that the government has broad authority to engage in warrantless, probable-cause-less searches of goods and people crossing the border, and that the same authority should apply to information crossing the border (as some lower courts have indeed held as to information crossing the border on computer disks), and

2. Keith itself expressly left open the question whether the Fourth Amendment rules applicable to purely domestic intelligence surveillance even applies to surveillance aimed at ferreting out the activities of "foreign power[s]" (a term that could encompass foreign nongovernmental organizations as well as foreign governments), as oppose to activities of domestic organizations (the matter that the Keith Court stressed was at issue in that case).

For more on these two arguments, see Orin's post from last December, which I also excerpt below (but click on the link to the original post to get links to earlier cases):

On the whole, I think there are some pretty decent arguments that this program did not violate the Fourth Amendment under existing precedent. There are a bunch of different arguments here, but let me focus on two: the border search exception and a national security exception. Neither is a slam dunk, by any means, but each are plausible arguments left open by the cases.

The border search exception permits searches at the border of the United States "or its functional equivalent." United States v. Montoya De Hernandez, 473 U.S. 531, 538 (1985). The idea here is that the United States as a sovereign has a right to inspect stuff entering or exiting the country as a way of protecting its sovereign interests, and that the Fourth Amendment permits such searches. Courts have applied the border search exception in cases of PCs and computer hard drives; if you bring a computer into or out of the United States, the government can search your computer for contraband or other prohibited items at the airport or wherever you are entering or leaving the country. See, e.g., United States v. Ickes, 393 F.3d 501 (4th Cir. 2005) (Wilkinson, J.).

As I understand it, all of the monitoring involved in the NSA program involved international calls (and international e-mails). That is, the NSA was intercepting communications in the U.S., but only communications going outside the U.S. or coming from abroad. I'm not aware of any cases applying the border search exception to raw data, as compared to the search of a physical device that stores data, so this is untested ground. At the same time, I don't know of a rationale in the caselaw for treating data differently than physical storage devices. The case law on the border search exception is phrased in pretty broad language, so it seems at least plausible that a border search exception could apply to monitoring at an ISP or telephone provider as the "functional equivalent of the border," much like airports are the functional equivalent of the border in the case of international airline travel. [UPDATE: A number of people have contacted me or left comments expressing skepticism about this argument. In response, let me point out the most persuasive case on point: United States v. Ramsey, holding that the border search exception applies to all international postal mail, permitting all international postal mail to be searched. Again, this isn't a slam dunk, but I think a plausible argument — and with dicta that seems to say that mode of transportation is not relevant.]

The government would have a second argument in case a court doesn't accept the border search exception: the open question of whether there is a national security exception to the Fourth Amendment that permits the government to conduct searches and surveillance for foreign intelligence surveillance. Footnote 23 of Katz v. United States left this open, and Justice White's conccurrence in Katz expanded on this point:

Wiretapping to protect the security of the Nation has been authorized by successive Presidents. The present Administration would apparently save national security cases from restrictions against wiretapping. We should not require the warrant procedure and the magistrate's judgment if the President of the United States or his chief legal officer, the Attorney General, has considered the requirements of national security and authorized electronic surveillance as reasonable.

The Supreme Court also left this question open in the so-called "Keith" case, United States v. United States District Court, in 1972. Justice Powell's opinion in the Keith case concluded that there was no national security exception to the Fourth Amendment for evidence collection involving domestic organizations, but expressly held open the possibility that such an exception existed for foreign intelligence collection:

Further, the instant case requires no judgment on the scope of the President's surveillance power with respect to the activities of foreign powers, within or without this country. The Attorney General's affidavit in this case states that the surveillances were "deemed necessary to protect the nation from attempts of domestic organizations to attack and subvert the existing structure of Government." There is no evidence of any involvement, directly or indirectly, of a foreign power.

The administration presumably takes the position that the President does have such power in cases involving foreign evidence collection, and that the NSA surveillance is such a case. The Supreme Court has never resolved the question, so it's an open constitutional issue. Nonetheless, between the border search exception and the open possibility of a national security exception, there are pretty decent arguments that the monitoring did not violate the Fourth Amendment. Maybe persuasive, maybe not, but certainly open and fair arguments under the case law.