In other words, many of the Northern states were attempting to change the constitution without going through the actual amendment process.
Their focus is on slavery, one could say they were obsessed with it, and potential impacts to that.
And as a side note, your man Rhett was threatening secession in 1856 if Fremont had won. And the primary benefit, as Rhett saw it, had nothing to do with taxes and everything to do with a resumption in the slave trade. Secession would, as Rhett saw it, "make Territories, now free, slave Territories, and to acquire new territory into which to extend slavery such as Cuba, North Eastern Mexico, &c but we would re-open the African slave trade that every white man might have a chance to make himself owner of one or more negroes, and go with them and his household goods wherever opportunity beckoned enterprise.