Paul Kiel at TPM Muckraker reports:

Senate Judicary Chairman Patrick Leahy (D-VT) and Ranking Member Arlen Specter (R-PA) have written Attorney General Alberto Gonzales to ask what power they think Congress has to restrict the waging of war.
(...)
"What constitutional authority do you recognize resides with the Congress with respect to war?" the letter asks. "How do you believe Congress can exercise its authorities? What limits to you believe exist on those authorities? We would appreciate your prompt reply and legal analysis."

Paul has posted a copy of the letter and a press release from Senator Leahy's office.