The phrase "ranking member" refers to the highest-ranking person from the minority party in a committee or group. They hold an important position and have influence but not as much as the leader from the majority party. Full definition
He is particularly known for his work on energy and law enforcement issues because he served as ranking member of the energy committee and spent more than 20 years as a police officer. (courant.com)
You should be a high ranking member of a commission somewhere. (sbnation.com)
The participants then had to report their levels of suspicion about a lower ranking member of the organization who took part in an ambiguous ethical violation. (sciencedaily.com)