long-standing Diet member
Refers to a member of the Japanese Diet (parliament) who has served for many years, often implying seniority or institutional memory.
As a long-standing Diet member, he contributed to the passage of many bills.
国会議員 is the general term for a Diet member, while 永年在職議員 specifically emphasizes long tenure.
Compound of 永年 (many years), 在職 (holding a post), and 議員 (assembly member).