I take issue with the way this debate is often framed: privacy versus security. It's misleading to suggest that privacy is directly opposed to security. A more apt name would be privacy versus surveillance. My point in these semantics is that, even given a wide latitude to monitor the people of this country, government surveillance doesn't necessarily make us any safer. The American people are and have been under what some would consider heavy surveillance for a few years and it has not demonstrably impacted our security. What makes us think that expanding the reach of that surveillance would suddenly be more effective? The likelihood of some bad actor within the government abusing their power to invade the privacy of American citizens, as has already happened with the NSA, is too great to justify whatever security may or may not be gained by giving that bad actor more tools to work with.
Secondly, regardless of the effectiveness of surveillance, privacy is a right. Plain and simple. By surveilling the American citizenry, the government violates that right on a national scale. I think that the right to privacy should be more clearly stated in the constitution, but it is alluded to in the fourth amendment, and it is clearly a principle on which this country was founded, even if the founding fathers didn't think of it in terms of privacy because this debate looked different due to differences between then and now in technology. The intent behind "no warrants shall issue, but upon probable cause" is pretty hard to mistake: it is a breach of privacy, and therefore a breach of our rights as citizens of this country to be subjected to involuntary surveillance by our government.