I certainly wouldn’t describe it as incomprehensible or even unlikely considering the lines that were clearly drawn by now, but he definitely was under no obligation to. It would have been an interesting Congressional fight had he not I would bet.
I don’t think FDR could have gotten a declaration of war against the Germans if Hitler hadn’t declared war first. The American population was still isolationist to a large degree, and the surprise attack at Pearl Harbor enflamed them against the Japanese, not the Axis.