"Consistent application of principles" is 1) reductionism to find them then 2) constructionism to apply them.
But "principles" is a plural and principles can collide with each other. Enter, rigorous use of critical thinking.
Shall we apply morality and blow up the world or is it more moral not to blow up the world and apply the other in safer times?
The primary job of an American President is the physical protection of the United States and its citizens.
Since Rand wasn't President she was free to get off on the morality, but not to prescribe and apply policy. Kennedy was free in the context of his duties to go to the ballet. Rand stayed clean and Kennedy got dirty. Politics is a very dirty business. Much dirtier today then way back then.
Trump is dirty