PRISM has helped thwart more than 50 terrorist acts worldwide, NSA director Gen. Keith Alexander has said.
But CNN terror analyst Peter Bergen doesn't think such broad surveillance has helped foil significant plots on U.S. soil. Normal police work has done the job in the overwhelming majority of cases.
PRISM's supporters and critics are divided between both parties, creating unlikely alliances between Democrats and Republicans to defend or condemn PRISM.
Republican House Majority Leader Eric Cantor promised "serious investigations into potential wrongdoing" over the program, while his party colleague, Rep. Peter King of New York, said journalists involved in reporting stories about the surveillance programs should be investigated for violating national security.
Snowden has fled to Hong Kong. Lawmakers in Washington are building a criminal case against him.