Final answer:
The concept of abolitionism fueled the debate over slavery during Manifest Destiny, as the expansion of U.S. territory raised moral and political issues about the extension of slavery into new states.
Step-by-step explanation:
The concept that fueled the growing debate over slavery in the United States during the period of Manifest Destiny was abolitionism. As the nation expanded westward, the debate over whether new territories should be free or slaveholding intensified. Manifest Destiny was the belief that the United States was destined to expand across the continent, which was justified by a sense of mission, cultural superiority, and divine ordination. However, as Americans moved west and encountered different societies and ways of life, the question of whether these new areas should perpetuate the institution of slavery became a critical issue.
Proponents of abolitionism argued that slavery was immoral and contrary to the principles of freedom and democracy that were foundational to the United States. The push for westward expansion brought up complex questions about the nature of freedom, republicanism, and the balance of power between free and slave states. These discussions would eventually lead up to the Civil War as the nation grappled with these fundamental conflicts.