The question remains, did he take it up out of some personal conviction, or as you later said, on behalf of his primary constituencies? The latter seems the very definition of opportunist, whereas the arguments here (by the pro-Lincoln crew) are always in favor of the former.
I would tend to give him credit for being aboveboard about his political interest; he was, after all, a professional lawyer and politician (not, however, a "railsplitter").
I'm not saying he was never opportunistic, but I think his embrace of the slavery issue -- or more to the point, the Kansas-Nebraska Act and its corollary theory of popular sovereignty (a.k.a. "squatter sovereignty"), which were topical at the moment he came off hiatus in his political career and began running for the Senate, was on the square.
He saw an issue and a need, and he offered his services as a legally-trained politician.
Later on was another story, of course.