What fears motivated nativists during the mid - to late - 1800s? What goals did they hope to achieve?

From the decade of 1820 the massive arrival of Irish and German Catholics was incubating a malaise that culminated in the mid-nineteenth century with the emergence of the American Party, known as the Know Nothings (I know nothing, the answer they should give if they were asked by the authorities).

The Know Nothings were grouped in numerous semi-clandestine organizations that vehemently opposed the Catholic invasion, reasoning among other things that they were vassals of a foreign prince (the Pope) opposed to the Enlightenment and therefore could not be good citizens of the republic.

The political proposals of the American Party consisted of measures against immigration, including the veto of immigrants to stand for elections and the requirement of 21 years of residence to obtain citizenship. The motto was summarized in 'only Americans will rule America'. The party reached several mayorships, control of the Massachusetts Assembly and 43 seats in the federal Congress. In 1856 his presidential candidate, Millard Fillmore, won 22% of the vote.