As in, when was such a state reached where people acknowledged that only a spark was needed to ignite an inevitable war?
/u/barkevious addresses some of your concerns in 'To what extent did the voters in the Presidential election of 1860 believe they were voting to on keeping the country together/out of a civil war?'
/u/freedmenspatrol also discussed secession and the election of 1860, where Republicans were the "third party".
You may be interested in the Civil War section of the AskHistorians booklist, which features several of the books cited by freedmenspatrol.