How did the political system fail to contain slavery as a political issue following the Mexican war?

A big factor was whether slavery was to exist in the newly formed western states and in the territory acquired in the American mexican war. The south of course wanted slavery to exist there and to have pro slavery senators and representatives in those new states to assure that any legislative means to abolish slavery would be blocked. However it became apparent that much of the newly formed states would not favor slavery. The south anticipated that they eventually would be the minority in congress. Moreover with Lincoln and the republicans in power it became evident that congress may soon abolish slavery. Also slavery was essential to the south's economy and cotton became a huge industry. Wealthy land owners would not allow the north to abolish slavery. Also there was still a romantic image of war and both sides believed the war would be over after a few battles. Apparently they were wrong.