As a Christian, I think you make a good point. Americans identifying as Christians has been on a decline for 60 years.
In 2020 it was 64% (90% in 1970), but a much smaller percentage admitted to regular church attendance. And certainly, folks that would be considered “Christian Nationalist” is a minority in that 64%. Statistics show that over two-thirds are Baby Boomers, who are nearing the end. Gen X (and younger) Christians tend to separate their spiritual and political lives much better than Boomers.
As a Christian, I’m more concerned with the damage Christian Nationalism does to the spread of the Gospel than American politics. Non-believers see Jesus through the lives and actions of His followers. If you profess to be a Christian and are not exhibiting Fruits of the Spirit (as defined in Galatians 5:22-23) then you are driving people away.
Christian Nationalism focuses on trying to legislate morality (legalism) and less on being “fishers of men” through love and forgiveness. Legalism has always driven folks away. Just ask the Pharisees.