Final answer:
Slave-owning states feared that adding non-slave states would result in losing political power in the Senate, compromising their ability to regulate the expansion of slavery, which was tied to economic and political control. The correct answer is option a.
Step-by-step explanation:
Politicians from slave-owning states feared adding non-slave states to the United States primarily because they were concerned about losing political power in the Senate, which is reflected in option A. While slave states held a majority in the Senate in 1846, they could not compete in the House of Representatives due to the North's rapidly growing population. If Congress had the power to legislate on the status of slavery in the territories, it could then potentially influence the status of slavery in the states themselves.
Factors such as the Northwest Ordinance and the Missouri Compromise set precedents for Congressional authority over territories, which meant that the balance of power in the Senate directly affected how slavery could be regulated in the nation.
The expansion of slavery was more than a moral issue; it had significant economic and political implications during the westward expansion of the United States. Acquiring new territories like California and New Mexico from the Treaty of Guadalupe Hidalgo heightened debates around whether these regions should be slave or free, eventually contributing to sectional tensions that made compromise increasingly challenging, as exemplified by the Compromise of 1850, the Fugitive Slave Act, and the Kansas-Nebraska Act.
Ultimately, the election of Abraham Lincoln, who was against the spread of slavery, precipitated the secession of southern states and the onset of the Civil War.
The correct option in the final answer, reflecting the historical events leading up to the Civil War, is A. Slave states feared losing political power in the Senate.