[Liz]

The Know Nothing Party was a 19th-century US political party primarily known for
its strong anti-immigrant and anti-Catholic sentiments, often referred to as “nativist” views.

They were named “Know-Nothing” because they would claim to “know nothing”
about their party’s secretive activities when asked about them.

[ansel12]

There was no such party with that name, their actual name was the “American Party”, originally the “Native American Party”, never the “Know Nothing Party”.

Can you think of any other example where the actual name of a political party is erased so effectively that people argue that the insulting nickname is their real name?

[GOPJ]

They were named “Know-Nothing” because they would claim to “know nothing” about their party’s secretive activities when asked about them.

Sounds like a fit.