Civic nationalism is the hope that you can have the beneficial aspects of a nation without real nationalism — that you can replace natus, the connection to the land and its people through birth, with patriotism and civics.
America gave it a pretty good shot in the 1950s. They had probably the best possible combination of circumstances for it: low immigration, the patriotism that followed winning a war, Hollywood having helped to spread that patriotism and sense of national sacrifice for the common good, a booming economy, and every school child dutifully reciting the Pledge every morning. All the elements were in place, and it lasted what, maybe a decade, before they started tearing it apart?
Nationalism comes naturally, by birth. Civic nationalism, as an attempt to create nationalism artificially, doesn’t. It’s not really nationalism, and before long, it’s not very civic.