“That would require a constitutional amendment, something he often called for and worked to pass once the election of 1864 gave him the congressional majority that it would require.”
Lincoln could have proposed a constitutional amendment to abolish slavery in his first inaugural address, demanded a vote, and if successful, ended slavery without a shot being fired. And he would have too, if it had been in the economic and political best interest of his backers.
In fact, Lincoln himself could have introduced an amendment to abolish slavery much earlier when he actually served in Congress. And he would have too, if it had been in the economic and political best interest of his backers.
Without a shot being fired.
Considering seven states had already seceded before he even took the oath, and he was hoping to ease their fears and advert a war, and that four of those seven states had declared in their articles of secession that they had done so to preserve slavery, it would have been pretty foolhardy to propose an amendment to end slavery at that point.