An important facet of Christian Nationalist ideology is the principle that God selects the nation's leaders. When leaders are good, it's because God wants them to lead the nation to victory. When leaders are bad, it's because God wants them to lead the nation to defeat as punishment for its sins. The idea that a nation's leaders are divinely selected is probably as old as religion itself. It allows for tighter integration of religious and political institutions, with religious figures backing the leader's claims to divine election and the leader backing the religious figures' claims to divine authority.
If people believe their leader is placed in charge by gods, they are less likely to question, challenge, or oppose his decisions. This is what makes such beliefs popular with authoritarian, totalitarian, theocratic, and fascist rulers; it's also what makes such beliefs inimical to democratic systems. Democracy requires the principle that citizens, not gods, choose their leaders and that the government is founded on human reason rather than divine agency. Both the American Constitution and the Declaration of Independence were written under a conscious rejection of Europe's long-standing belief in the Divine Right of Kings -- the idea that monarchs owe their rule to the will of God rather than the will of the people.
Unfortunately, over 200 years of democratic tradition have failed to extinguish the religious impulse to attribute divine agency to democratically elected leaders. There are many who believe God is responsible for George W. Bush being president -- including, it seems, George W. Bush himself.
This is a problem because if God, not the people, is the sovereign power responsible for Bush being president, then it means Bush is ultimately responsible to God rather than to the people. Bush's job becomes doing the will of God, at least as he interprets it, rather than the will of the people or to serve the interests of the people. This is fertile ground for Christian Nationalism and Christian Fascism because it allows for the excision of democracy, democratic elections, the separation of powers, constitutionally protected rights, and everything else which makes America a secular and free nation.
The above image was created from a World War I poster describing how the "Victory Fund Campaign" (and presumably religious faith) "sustained him" and "supported the war-time agencies that helped him."