Congress, public opinion, and an informal constraint on the commander-in-chief

US presidents have routinely ordered the use of force without seeking prior authorisation from Congress. However, this practice does not mean that the legislature is irrelevant, as Congress often influences decisions by exercising informal political levers. One of the most important is through Congress' ability to affect popular support for the commander-in-chief. Through a pair of experiments embedded on nationally representative opinion surveys, this article evaluates whether Congress' constitutional prerogatives in war powers remain relevant when battling the president in the public sphere. Policy criticism significantly decreased support for the use of force, as did challenges to administration actions on constitutional grounds. Although Congress routinely fails to use the constitutional tools at its disposal to check the commander-in-chief, these powers bolster Congress' capacity to influence public opinion. Hence, while presidents enjoy considerable leeway in the military arena, Congress' capacity to erode public support can serve as a check on presidential power.