A reader writes:
The abolitionists certainly did accuse Lincoln of moral equivalence. Even the less radical anti-slavery men, like Horace Greeley, accused Lincoln of not taking the moral cause of slavery seriously enough as a component of the war. Consider Greeley's famous letter to Lincoln pleading with him to make the war a moral cause. Lincoln's reply to Greeley is very telling. He specifically separates the idea of personal conviction of morality (his "often expressed wish that all men, everywhere, could be free") with his offical morality as to his position as President. In his offical capacity, Lincoln is, of course, famously concerned not with slavery, but preserving the union ("If I could save the union by freeing some..."). But more telling is his attitude of doubt and his desire to change course when the facts merit:
"I shall try to correct errors when shown to be errors; and I shall adopt new views so fast as they shall appear to be true views."
That, to me, is the heart of Lincoln's anti-fundamentalism: his willingness to consider opposing viewpoints and to adjust when times call for it. And it was what separated him from both the abolitionists and the southerners.
And it is what made him a conservative genius - on a par with Elizabeth I, James Madison and Benjamin Disraeli. You can read more about the Lincoln-Greeley relationship here.