The warrantless wiretapping of citizens and residents of the United States is flatly illegal. It is a violation of criminal laws and an impeachable offense. No doctrine of Executive power or claim of authority to use military force can possibly authorize wiretapping that violates the core protections of the United States Constitution. Wiretapping is governed by statute and can only be carried out in the United States in compliance with the statute. Warrantless wiretapping against citizens and residents of the U.S., even when authorized by the President is a crime. It violates 18 USC sec. 2511, and those convicted can be sentenced to 5 years for each violation.
http://www.ccr-ny.org/v2/reports/report.asp?ObjID=YfrbEeEnYL&Content=685