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.
either would DO/SAY ANYTHING to get ahead. ANYTHING!
BOTH were guided by SELF-interest. nothing more/nothing less.
free dixie,sw