The Democratic-Republican Party had split before the decline of the Whigs and was not a factor in their downfall. The Whig Party struggled with internal divisions over economic policies and stances on slavery, and was further challenged by new parties focusing on immigration and the slavery issue. Hence the correct option is C.
The factor that was not a significant reason for the eventual downfall of the Whig Party in the United States is c) Emergence of the Democratic-Republican Party. The Whig Party did struggle with internal divisions, particularly over economic policies and their inability to form a coherent stance on slavery. These divisions included debates over national resources, whether they should be allocated to economic and mercantile development or to promoting populist issues.
Additionally, the Whig Party was harmed by their indecision during the rising concern about immigrants and slavery. The growing party realignment in the 1850s included new parties focused on these issues, leading to the rise of the Know-Nothing Party and the Republican Party, which further undermined the Whigs. However, the Democratic-Republican Party had already split long before the decline of the Whigs, which occurred after the 1852 election. Thus, the emergence of the Democratic-Republican Party is not associated with the downfall of the Whig Party.
Hence the correct option is C.