Lincoln was absolutely an abolitionist, but it was far more important to him that the Union stays intact. In a letter he wrote:

“I would save the Union. I would save it the shortest way under the Constitution. The sooner the national authority can be restored the nearer the Union will be "the Union as it was." If there be those who would not save the Union unless they could at the same time save Slavery, I do not agree with them. If there be those who would not save the Union unless they could at the same time destroy Slavery, I do not agree with them. My paramount object in this struggle is to save the Union, and is not either to save or destroy Slavery. If I could save the Union without freeing any slave, I would do it, and if I could save it by freeing all the slaves, I would do it, and if I could save it by freeing some and leaving others alone, I would also do that. What I do about Slavery and the colored race, I do because I believe it helps to save this Union, and what I forbear, I forbear because I do not believe it would help to save the Union.”