If you know your history you will know that Jeanette Rankins was the only person who denied the US to go to war after December 7th, 1941. In order for the US to go into any kind of war I think that they should need to get congress to accept the Declaration of War before you go to war itself. However as world tension increases more votes can easily get secured and when it reaches 100% you will have the highest chance to roll enough votes to go to war. However if the US is attacked you will have a 100% chance to go to war unless you have 98 Jeanette Rankins in your Senate.
If you know your constitution the President doesn't need 100% of the votes. Nor was Rankin a senator - she was in the House of Representatives, which voted 388-1. The Senate voted 82-0 (members must have been missing because of the short notice for the vote).
Nor is there a "roll" like some random lottery result. As WT increases, opinion in Congress moves towards the 50% votes required to allow the President to prosecute the war, as the public increasingly accept the need to confront Germany and Japan. He certainly doesn't need to convince any Rankin's (she was a pacifist). In 1941 there was hardly likely to be a 100 pacifists elected by the US people.
Plus IMO the vote in Congress was a formality. Japan had already declared war. The President as the Supreme Commander had sufficient powers to fight the war. The vote is part of a PR exercise to show the world that the USA was overwhelmingly united in their will to defeat Japan, and especially that the President had the support of both political parties.