The 19th century was a time of rapid population growth in the United States, and much of it was due to immigration from Europe. In the 1840s and 1850s, the largest proportion of immigrants came from Ireland and Germany, and most were Catholic. The Germans spread across small communities as far west as Wisconsin and Texas, but the Irish concentrated in the larger cities on the eastern seaboard, especially Boston, New York, Philadelphia, and Baltimore. Local third- and fourth-generation Protestant immigrants from England resented the new arrivals and organized “Nativist” associations. Among these was the anti-Catholic American Party, better known as the Know Nothing Party, which enjoyed spectacular success in Massachusetts and other states during 1854–1855. But, by 1862, the party was dead. This article examines how moral panic theory, the theory of persistent cultural patterns and cycles, and revitalization theory may offer insights into the Know Nothing Party. Each of these theories explains both the emergence of the party and its rapid demise, and suggest that each can make a contribution to understanding anti-Catholicism in nineteenth-century America, and the Know Nothing Party in particular.