As I mentioned to you in a note, I agree that this was Lincoln's motive in taking up the cudgels against slavery. Donald's biography of Lincoln (Lincoln, 1999) adds further evidences from correspondence of Lincoln's motivation, but this quote is expository.
The success of abolition, and its war, was sustained by a Party. The business of finding out which motives for the abolition of slavery and the waging of that war were preponderant and effectual in the coalition of interests and reasons belongs to the historian. Unfortunately, McPherson offers us the aspect instead of a polemicist.
It's too bad the Census of 1860 couldn't have run an excimiating poll on the subjects of slavery, secession, and maintenance of the Union with its "long form" of the day. Then we should know a lot better what we are talking about when we start ascribing motives to large, kaleidoscopic masses of politically active people.
That is a problem. I've looked around for stats on the subject estimated by various persons and studies during the time. One of the most detailed I've seen from before the war was in a speech by John Calhoun given in 1847. He breaks down the northern population by rough estimates and supports those estimates with statistics about various causes and their electoral strengths in previous elections.