Why did the southerners fear the Republican control over the US government as a threat?

In the Presidential election of 1860, Abraham Lincoln contested as a candidate of the Republicans, and Stephen Arnold Douglas stood as a Democratic candidate. Abraham Lincoln stood for the freedom and emancipation of slaves, while Douglas wanted slavery to continue. The southerners feared that if Lincoln became the President, he would keep his promise to stop the expansion of slavery. Many southerners thought that even if Lincoln did not abolish slavery, northerners would do so and therefore it was time to quit the Union.