2. why did southern politicians begin talking about secession?

The Southern politicians started to think and talk about secession because of multiple reasons, from economic to social ones. The Southern politicians did not had lot of things in common with the Northern politicians, nor did their respective states.

The Southern states were economically stronger. They had a very large agricultural production and large market to sell it, while the North was reliant on other means. The Southern politicians wanted to keep the things as they are socially, while the Northern ones wanted lot of changes. The slavery was also a major issue, as the South was seeing the slaves as a must for their economic prosperity, while the North wanted to abolish the slavery.

In general, the South and the North were like a cat and dog, pretty much disagreeing in everything, having different attitudes and approaches to pretty much everything. This led the Southern politicians to talk seriously about a secession as they were not seeing a future together with the North as they were radically differing from one another.