Final answer:
The Know-Nothing (American) Party and the Republican Party emerged in the 1850s to fill the void left by the declining Whig Party.
Step-by-step explanation:
The two new parties that filled the gap left by the demise of the Whig Party in the 1850s were the Know-Nothing Party, or the American Party, and the Republican Party. The Know-Nothings gained traction by opposing immigrant influences, particularly during a time of increased immigration in the late 1840s and early 1850s. In contrast, the Republican Party began to amass support by focusing on concerns related to the expansion of slavery and its impact on non-slaveholding whites. The tension between these two concerns, as seen in the violence in Kansas and political events such as the assault on Charles Sumner, eventually set the stage for the 1856 presidential contest. This positioned the Republicans to eventually overtake the Whigs as the second major party in the United States.