I heard at one point that that North was mostly willing (at least in the sense of not getting up the energy to do anything about it) to let the South keep slaves, until the South managed to get the Feds to force Northerners to hunt down escaped slaves.

This interpretation (and I'm not qualified to judge its validity) makes the situation all the more ironic - the Union got involved because the Rebels (not Confederate yet, IIRC) were trampling on state's rights.