Which event made slavery a central political issue in the mid-1850s?

The choice to enable the general population of Kansas to vote whether their new state would be slave or free. 
This meagerly populated domain pulled in all way of cross-fringe looters from the two sides of the verbal confrontation, resolved to threaten voters and get the outcomes pronounced invalid. 
There was horrible slaughter, and demonstrate couldn't help suspecting that the bondage level headed discussion could never be settled, with the exception of through battle. The slave-question that caused the new Compromise of 1850 was the confirmation of California as free soil.