First of all, neither “Christians who attend church” nor “Christians to don’t attend church” are ethnic groups. They are demographic groups.
But to answer your question, it doesn’t make sense to expect Christian nationalism to map cleanly onto either of those demographics when no other other group of Christians does. After all, nearly 40% of Americans consider themselves to be very religious despite the fact that only 20% attend weekly services. And about half of the non-voting population nevertheless identifies itself with one of the two major political parties. In short, identity and participation don’t necessarily go together.
We should therefore expect Christian nationalists to be split among those who regularly attend church and those who don’t. And there could be all sorts of reasons. Some might pick up their nationalist views at church, which means regular attendance may strengthen those views. But others might go to church because they think that behavior fits with their preexisting identity (and may continue attending despite their church not supporting—or even actively opposing—their nationalist views.
On the flip side, some might avoid church because they don’t feel supported there, while others might not attend simply because they think their self-identification and political activities are enough. Ultimately, Christian nationalism is a political ideology at least as much as it is a religious ideology. And political ideologies can spread with or without church involvement. So while some churches might promote such views, they will never be the only source of them.
All that said, I did find this analysis from Robert P. Jones (a graduate of Southwestern Baptist Theological Seminary and the founder of the Public Religion Research Institute). This study finds that “church attendance has no significant impact on the relationship between white Christian identities and holding racist views,” but also that “an increase in racist attitudes independently predicts an increase in the likelihood of identifying as a white Christian, and identifying as a white Christian is independently associated with an increased probability of holding racist attitudes.”
The main takeaway for Jones is that church attendance doesn’t mitigate racist or anti-democratic sentiments and is in some cases an overlooked source of such views. This is not great news for those who want their churches—and the Christian church more broadly—to be solely an agent of good in the world, but it can also be an opportunity for those same people so long as they are able to recognize that the solution to these problems has to start at home.