I think you made this up, and that no one has actually made this claim that ---most--- Northerners "passionately" opposed slavery. This is a total straw man. What really matters is any opposition to slavery, and it does seem like the vast majority of Northerners were opposed to slavery. Why does it matter why? The only thing that matter are actions and results. In 1856, a supermajority of Northern states (11/16) voted for the Abolitionist presidential candidate Fremont - That's what really matters.