Why was the Know-Nothing Party formed?

The Know-Nothing Party (also known as the American Party)
was formed because of nativism.  Nativism is an ideology that holds
that the "native" people of a given country are superior to others and that those others
should be kept out of "their" country.  In the United States in the 1840s, this meant
that people of generally English descent felt superior to immigrants of German and Irish
descent.

During the 1840s, huge numbers of immigrants had
come to the US from Ireland and Germany.  "Native" Americans felt threatened by these
newcomers.  They felt that the immigrants (especially the Irish Catholics) were a danger
to American traditions and values.  In an attempt to fight against the immigrants and
their increasing presence in US, the Know-Nothing Party was formed.  As the link below
says, the party was made of people who

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...feared the foreign influence of immigrants ...
who were arriving in record numbers to the United States in the
1840s.