Here's your weapon; now shoot yourself in the foot
Offensive military action against Iran would be illegal under the United Nations Charter, which requires that members settle international disputes by peaceful means. The UN Charter is a treaty ratified by the US and thus part of American law under the Supremacy Clause of the Constitution. Under the Charter, a country can attack another only in self-defense or with the blessing of the Security Council....So, prog caucus -- get busy.
Congress should immediately pass a binding resolution reaffirming the United States' legal obligations and informing the Bush administration that it will not concur in any invasion or military action against Iran, would refuse to approve any funding for it, and would consider actions taken in contravention of the resolution as impeachable offenses.