Stephan A. Douglas , Democrat, was probably the most flexible on the slavery question. He proposed popular sovereignty -- letting the people of a state or territory decide by an election whether of not to allow slavery in their state.
An American state where slavery was not allowed by law was called a "free state." These states did not permit the practice of slavery within their boundaries, in contrast to states where slavery was legal.
Lincoln argued that it was wrong to decide whether to allow slavery in a state or territory by voting
Popular Sovereignty
The principle of letting the people of a new state vote on whether it was to be a slave-state or a free state.
Kansas
The slavery debate was mostly to do with the new states - whether they would be slave or free, the proportion of slave-states in Congress, and their influence on the voting.
Stephen Douglas believed that the issue of slavery should be determined by popular sovereignty, allowing individual states and territories to decide for themselves whether to allow slavery. He supported the idea that each state should have the right to choose whether to be a free or slave state.
The solution was, to divide Clay's plan into a series of measures that Congress could vote on separately
It is a free STATE that is closed by slavery. You know..... like when there is a free state, there is NO slavery. Therefore, it is like slavery is closed, that is way it is called closed to slavery.
The decision of whether to allow slavery or not in each state was left for the residents to decide.
Popular Sovereignty