The correct answer is "territorial expansion raised the question of whether new lands should be free or slave."
Slavery became more central to American politics in the 1840s in that territorial expansion raised the question of whether new lands should be free or slave.
Let's set the example of the acceptance of California as a state on September 9, 1850. On this day, California became the 31st state of the Union and entered as a free state. A vast portion of the former Mexican territory was gained by the United States as part of the negotiations of the Treaty Guadalupe-Hidalgo. California, Utah, Arizona, and New Mexico were incorporated into the US. Slavery really divided the American people. The northerners supported abolition while southerners supported slavery.
