What was the major effect as a result of the Kansas-Nebraska Act?

The Kansas-Nebraska Act allowed each territory to decide the issue of slavery on the basis of popular sovereignty. Kansas with slavery would violate the Missouri Compromise, which had kept the Union from falling apart for the last thirty-four years. The long-standing compromise would have to be repealed.

Explanation:

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The Kansas–Nebraska Act of 1854 was a territorial organic act that created the territories of Kansas and Nebraska. It was drafted by Democratic Senator Stephen A. Douglas, passed by the 33rd United States Congress, and signed into law by President Franklin Pierce. Douglas introduced the bill with the goal of opening up new lands to development and facilitating construction of a transcontinental railroad, but the Kansas–Nebraska Act is most notable for effectively repealing the Missouri Compromise, stoking national tensions over slavery, and contributing to a series of armed conflicts known as "Bleeding Kansas".

Explanation:

However, one of the most significant and lasting effects the Kansas-Nebraska Act had on the American political system was the formation of the Republican Party. The Kansas-Nebraska Act directly led to the  creation of the Republican Party.