I am not sure, off-hand, about actual numbers of abolitionists in 1860. I'll check into it a bit. Every study I can think of that covers American racial views prior to the Civil War seems to agree that abolitionists were a minority.

Yet, we can still make the claim that "most" Northerners were not abolitionists based on other political realities. Northern Democrats were quite powerful, and there were few if any abolitionists in their ranks. They were the "conservative" party of the time.

And we know that before the war, most Republicans were not true abolitionists either. Though some abolitionists joined the Republican Party, there were conservatives and moderates within that organization that tempered the more radical ideas and interests. Lincoln was a moderate, and even Seward (who was considered more radical) was not a true abolitionist. Eric Foner's study "Free Soil, Free Labor, Free Men," is a great book about the Republican Party's position (or positions) on slavery.

So, overall, it is safe to say that abolitionists only made up part of roughly one half of the Northern political environment. They were, thus, a minority.