Sentence:  Taking account of the fact that such a move on our part would be unpopular in world opinion , he argued that the responsibility of the United States is `` to do , confidently and firmly , not what <WSD>is</WSD> popular , but what is right '' . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)