I agree. Broad sentiment in congress was not in favor of US entry into the European War, and I doubt that the death of a US cabinet official due to the sinking of an Amercan ship would unite sentiment in the same way Pearl Harbor did. Depending how the Germans played their cards (ie if they were not suitably apologetic) an act such as this might allow FDR to get congress to approve a declaration of war against Germany, but it would be a strongly divided vote. The war would be broadly unpopular with a sizeable segment of the American people, and politically, the US would probably be far less willing to join a grand alliance against Hitler including the Soviet Union. I also believe US political realities might force the Administration to wage this as a more limited war (ie a naval and air campaign against Germany, logistic cooperation with Britain, but with no immediate committment to a major US role in an invasion of Europe, and to be more open to a negotiated peace or armistice, rather than unconditional surrender, especially if things look bad for the allies.
Also, maybe I am seeing things through post-Vietnam glasses, but there might always be some suspicion that the causus belli was a set up engineered by the Adminsitration and Britain.
Also, how do you butterfly away Japanese ambitions? Once at war and on a war footing, the US might find it easier to declare war on Japan for any number of aggressions, even if this did not involve a direct attack on US soil or personnel. Once that happens, there may be no "Germany first" policy, and most US effort may go against the Japanese - who after all were seen as America's main strategic threat in areas were the US has a vested interest