I think many people forget that Lincoln was a politician. Politicians by nature try to build the largest coalition possible to enable them to win in an election. At the same time one has to remember during the Lincoln-Douglas debate:
1) Douglas explicitly stated, "I am opposed to taking any step that recognizes the Negro man or the Indian as the equal of the white man."
2) Lincoln did express some white supremacist ideas during the debates because he was campaigning in Southern Illinois, but compared with Douglas he was by far more on the side of equality.
3) I cannot state this better than the book, Lies My Teacher Told Me:
Lincoln's ideas about race were more complicated than Douglas's, however. The day after Douglas declared for white supremacy in Chicago, saying the issues were "distinctly drawn," Lincoln replied and indeed drew the issue distinctly:
I should like to know if taking this old Declaration of Independence, which declares that all men are equal upon principle, and making exceptions to it--where will it stop? If one man says it does not mean a Negro, why does not another say it does not mean some other man? If that Declaration is not ... true, let us tear it out! [Cries of "no, no!"] Let us stick to it then, let us stand firmly by it then.
Those hardly sound like the words of someone who wasn't having some deep thoughts about equality, but I agree he didn't vigorously proclaim it explicitly from the mountaintop while campaigning for President the first time. His ideas on race grew during his Presidency, and this is one of the great reasons he was such a good President.