I think it is important for you write your own thesis statement, because then it belongs to you...something you can be proud of. However, I can give you a few suggestions that will help you in your task.
1. Read The Lincoln-Douglas debates. Since your paper must express Lincoln's perspective pay attention to his responses in the debate.
2. Research Lincoln's childhood, it was not uncommon for him to witness slaves chained together passing by his home as a young boy. Although subtle, these memories had an impact on him as an adult.
3. Look at his politics during the 1860 Presidential election. His views were clear, slavery was wrong however he did not advocate its immediate abolition just the spread of the institution of slavery into the west. (He was well aware of The Missouri Compromise, The Compromise of 1850, and The Kansas-Nebraska Act of 1854. The first two were nothing more than a band aid on a wound that refused to heal, the latter proved deadly)
4. 1863-The Emancipation Proclamation speech delievered by Lincoln freed all persons in bondage in the states of rebellion. First, understand the President has no legal authority to do this, however the speech was heard and printed in newspapers across the nation. His speech was successful because he sprinkled morality over the tug of war over power between the federal and state governments. Lincoln's primary goal was to keep the union together.
5. After you research one or all of these suggestions, write your statement.
Good Luck !!