Final answer:
Abraham Lincoln joined the Republican Party after the Kansas-Nebraska Act passed, sparking a fight against slavery.
Explanation:
Abraham Lincoln ended his law career and joined the new Republican Party when the Kansas-Nebraska Act passed. This act overturned the Missouri Compromise of 1820 and allowed new territories to decide on slavery, which mobilized early Republicans to fight against slavery. The Republican Party emerged in the 1850s as a coalition of various anti-slavery groups, including ex-Whigs, anti-slavery Democrats, and Free Soilers, who joined forces to combat the spread of slavery.
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