Should Christians seek high political office to lead a nation? Should adherents to Christianity rise to leadership through their light of righteousness and humility?
If I can take a stab at it, I'd define "Christian nationalism" as the political project where a minority of the population attempts to enact their preferred social policies by explicitly arguing for a "greater good" or otherwise divinely provided policy guidance at the expense of normal democratic processes and procedures, where the religious appeal and justification would be unnecessary since the proposed policies would already enjoy popular support. It's attempting to add democratically-unaccountable religious figures into the decision-making process solely based on their religious positions (or beliefs) and not because a democratically-elected figure appointed them.
It's only "Christian" in America because that's largest minority arguing that our current democratic processes are broken and the (classical) liberalism that informs our system of government only works when enough Americans are Christians themselves, and that the State has a duty and obligation to step in to ensure that the United States "remains" a "Christian nation". It's fundamentally the same idea as rule justified by the divine right of kings, but with those arguing for it believing that they are one of the anointed folks who God thinks should be calling the shots.
Voters should never expect politicians to meet their morality standards. Elected officials are there to govern legally in accordance with the Constitution.