Proposition nation: A nation founded on certain creeds or ideals [1]. A.k.a. a "creedal nation" [2].
You probably heard it called "civic nationalism", which uses different words to say the same. From wikipedia [3, emphasis mine]: a "civic nation" defines itself not by culture but by political institutions and liberal principles, which its citizens pledge to uphold. Membership in the civic nation is open to every person by citizenship, regardless of culture or ethnicity. Those who share these values are considered members of the nation
[1] https://en.wiktionary.org/wiki/proposition_nation - Barbara Perry, 'Hate Crimes' (volume 3, page 117): Because they view the world through a racial lens, white nationalists do not agree with the notion that America is a creedal or 'proposition nation', meaning one whose societal and cultural norms are based on acceptance of the Declaration of Independence and the other founding documents...."
You probably heard it called "civic nationalism", which uses different words to say the same. From wikipedia [3, emphasis mine]: a "civic nation" defines itself not by culture but by political institutions and liberal principles, which its citizens pledge to uphold. Membership in the civic nation is open to every person by citizenship, regardless of culture or ethnicity. Those who share these values are considered members of the nation
[1] https://en.wiktionary.org/wiki/proposition_nation - Barbara Perry, 'Hate Crimes' (volume 3, page 117): Because they view the world through a racial lens, white nationalists do not agree with the notion that America is a creedal or 'proposition nation', meaning one whose societal and cultural norms are based on acceptance of the Declaration of Independence and the other founding documents...."
[2] Why America Is a ‘Creedal Nation’ - https://www.wsj.com/opinion/why-america-is-a-creedal-nation-...
[3] https://en.wikipedia.org/wiki/Civic_nationalism