it was not southern democrats that introduced the amendment..it was the north trying to show protection of slavery...if "protection of slavery" was the only issue that forced slavestates out..they wouldnt have left.
I think timing is important here. By the time the Corwin Amendment was proposed, the ill fated secession plan of many of the slave states was already a fact. It was a last ditch effort of northerners who didn’t want war to stave off the impending disaster by capitulating to the Slave Power yet again in the name of peace. They were too late and didn’t understand that the Slave Power wanted war.