Sentence:  Meanwhile , after 24 years in the Senate , Rhode Island 's durable Democrat Theodore Francis Greene - having walked , swum and cerebrated himself to the hearty age of 93 - left that August body ( voluntarily , because he could surely have been re-elected had he chosen to run again last November ) , as the oldest man ever to <WSD>serve</WSD> in the Senate . 
Meaning of the word marked with <WSD> tags:  do duty or hold offices; serve in a specific function