Complex sentence: Mikulski is the longest-serving present Senator who has never been the chairperson of a full standing committee of the U.S. Senate; she is also the longest-serving/highest-ranking Democrat who is not a present chairperson of a full standing committee.
Split into simpler sentences: Mikulski is the longest-serving present Senator who has never been the chairperson of a full standing committee of the U.S. Senate. She is also the longest-serving/highest-ranking Democrat who is not a present chairperson of a full standing committee.