Take, claim, seize, etc. the moral high ground
Definition 1 of 2
Pro Tip 1/3Cultural Caution
The phrase can be viewed differently across cultures; some may see it as self-righteous. 
The candidate tried to take the moral high ground, but some voters saw it as arrogance.
Pro Tip 2/3Argument Strategy
Using the phrase in debates suggests one's stance is ethically better than the opponent's. 
She took the moral high ground, arguing that honesty is paramount in business.
Pro Tip 3/3Conflict Context
Most useful in situations where values are in conflict and ethical superiority is claimed. 
During the protest, the leaders claimed the moral high ground over the issue.