Final answer:
Native-born Protestant Americans predominantly feared that Catholic immigrants would politically, socially, and culturally overwhelm the Protestant ethos of America, as exhibited by the beliefs and actions of the Know-Nothing Party.
Step-by-step explanation:
Native-born Protestant Americans in the 19th century harbored a fear that the increasing number of Catholic immigrants, particularly those from Ireland and Southern Europe, would challenge the Protestant cultural and political majority. This fear was rooted in the belief that Catholics would not assimilate into American society, potentially remaining loyal to the Pope instead of adopting American values and institutions. This concern extended to the educational system, where Protestant Americans feared that government aid to religious (Catholic) schools would discourage assimilation and threaten their own cultural hegemony. The Know-Nothing Party epitomized these nativist sentiments, actively advocating for policies designed to diminish the political power and social influence of immigrants, such as extending naturalization periods and restricting public office to the native born.
The tension between nativist groups and immigrants was exacerbated by the fear of Catholics' potential to politically and socially dominate, leading to efforts to secure a homogeneous Protestant American culture and preserve the existing social order.