Sentence:  Mrs. Clinton's main opponent, Senator Bernie Sanders, a self-described Democratic Socialist, has proved to be more formidable than most people, including Mrs. Clinton, anticipated. He has brought income inequality and the <WSD>lingering</WSD> pain of the middle class to center stage and pushed Mrs. Clinton a bit more to the left than she might have gone on economic issues. 
Meaning of the word marked with <WSD> tags:  To stay or remain in a place or situation, especially as if unwilling to depart or not easily able to do so; to loiter.