John Schmidt, associate attorney-general during much of the Clinton years, reviews statutory language and court rulings on Presidential authority to order warrantless taps. An excellent read. Even better is a piece by polymath-federal judge Richard Posner, who explains that machine sifting of huge volumes of data actually minimizes invasion of privacy by removing collected data--the vast majority of that captured--before humans examine it. Posner adds that limiting surveillance of domestic citizens to those suspected of working with terrorists removes from scrutiny those who might be unwittingly aiding the bad guys.
"The goal of national security intelligence is to prevent a terrorist attack, not just punish the attacker after it occurs, and the information that enables the detection of an impending attack may be scattered around the world in tiny bits. A much, wider, finer-meshed net must be cast than when investigating a specific crime."
NRO's Andrew MccArthy, ex-prosecutor, lists the numerous permissible warrantless searches federal law permits today.
Schmidt: President Has Legal Authority for Warrantless Taps
Richard Posner: Our Domestic Intelligence Crisis
MccArthy: Warrantless Search List