Senators raised doubts about the legal rationale for the Bush administration’s eavesdropping program Monday, forcing Attorney General Alberto Gonzales to provide a lengthy defense of the operations he called a vital “early warning system” for terrorism.

A handful of Republicans joined Democrats in raising questions about whether President Bush went too far in ordering the National Security Agency’s monitoring operations. The senators were particularly troubled by the administration’s argument that a September 2001 congressional resolution approving use of military force covered the surveillance of some domestic communications.