I read an interesting letter on that subject the other day...
"Every just view that can be taken of this subject, admonishes the public of the necessity of a rigid adherence to the simple, the received, and the fundamental doctrine of the constitution, that the power to declare war, is fully and exclusively vested in the legislature; that the executive has no right, in any case, to decide the question, whether there is or is not the cause for declaring war; that the right of convening and informing congress, whenever such a question seems to call for a decision, is all the right which the constitution has deemed requisite or proper." - James Madison, Letters of Helvidius No. 1 (emphasis added)
Seems as if Madison saw things quite clearly on that issue. If the president wanted to engage in war he had NO CHOICE but to call congress and his power to convene congress was intended for exactly that purpose. How one could claim that Lincoln acted properly by waging war without Congress' presence is beyond absurd.