In the summer of 1941, FDR was engaging in ASW warfare against Germany (USS Greer Incident), in violation of the Constitutional requirement that the Congress declare war. (Which Presidents since Jefferson had done.)
In the late fall of 1941, Japan attacked the US without warning, precipitating a Declaration of War by the Congress on Japan. At least one Democrat voted "no". Congress did not declare war on Germany or Italy.
This pissed Hitler off, and he then declared war on the US (LINK) and the US Congress finally responded by issuing a declaration of war against Germany and Italy. Text of US Declaration of War against Germany - Note statement that Germany has declared war on the US.