he history of Israel as told in Kings and Chronicles shows that a wicked man will lead others into ways of destruction and death. It also shows that a good ruler will not by himself be able to bring the people closer to God unless they themselves can commit to obedience to Him.
But this does not mean the ruler bears no responsibility, even in a state like America without an endorsed religion. It is expedient and more that the ruler not lead the people astray, though he cannot by himself bring a people to repentance. There is a difference between not being the one with authority and power to change the wicked hearts of men (and knowing that fact) and setting a bad example for others to follow.
Failure to execute justice within, failure to not become moral jelly before the feet of oppressive regimes, failure to refrain from positive injustice is just that: failure. A failed state. And to that it makes no difference whether the state is like the biblical Israelite monarchy or a constitutional republic.
Obviously the policies of the state do matter, or public policy would be irrelevant. But there is much more going on in what the government does than force of taxation, force of conscription, force of just and unjust laws. It matters whether a senator has been divorced and remarried (Mal. 2.16; Mt. 5.31–32). It matters whether a senator has performed or called for a murder (Is. 1.21), including for his own child or his patient’s. It matters whether a senator has dishonoured his father and mother (Mk. 7.9–13).
The word Senate is derived from the Latin word senex, ‘old man, elder’. Like it or not, the Senate is supposed to be a body of the elders of the nation. So, too, with the President, that he, like a senator, is to be held to standards.
Christians cannot expect non-Christians to act in a Christ-like manner, but they can do what they can for the government not to mis-shepherd the people into murder, theft, greed, idolatry and all manner of abominations. What do you think Christians should do?