Why did Lincoln think the draft was constitutional and necessary

Abraham Lincoln was a pragmatist in the use of power. Winston Churchill wrote that Lincoln was "anxious to keep the ship on an even keel and steer a steady course, he may lean all his weight now on one side and now on the other. His arguments in each case when contrasted can be show to be not only very different in character, but contradictory in spirit and opposite in direction; yet his object will throughout have remained the same ... we cannot call this inconsistency. The only way a man can remain consistent amid changing circumstances is to change with them while preserving the same dominating purpose.