"However, you misunderstand the dominant Southern political sentiment during the election of 1860, which was Unionist."
You are correct that the South was NOT strongly secessionist in 1860, However the threat was on the table politically. I was primarily focusing on why Lincoln won the northern states votes by such margins. The question of "Preserving the Union" was not as much a factor in the southern vote since Lincoln wasn't on those ballots.
"The position being argued in Southern States was not secession, but slavery in the territories."
Again this is true, but not precise. The arguement was about the EXTENTION of the Constitution into the Territories. The arguement was like saying, 'Ok, the 1st Amendment will apply to New York, but it will not apply to South Carolina'. The issue of Slavery under the Constitution was a state issue, and should have been left up to the territories themselves, that was the southern arguement. It was in reality NOT a Federal issue for congress to regulate, constitutionally.
California was actually the sticking point because of the discussions over whether that state should enter the Union as one state, or two, divided as the northen half free and the south half slave. Once California entered as a single state, what of the other territories? That was where the 1820 and 1850 Missouri compromises, which were illegal anyway, broke down and you had Bleeding Kansas and the Kansas Nebraska act trying to put the genie back in the bottle.
No one wanted secession, BUT the possibility was very much a part of the thinking of both North and South in 1860.