Nexus is an approved travel document by US CBP and can be used instead of a passport.
Quote:
All Canadian Citizens entering the United States by AIR are required to present a valid passport, or a NEXUS card when departing from a designated Canadian airport.
By Land and Sea (including ferries) - Canadian citizens traveling to the U.S. by land or sea are required to present one of the travel documents listed below, and may generally visit the U.S. for up to six months. CBP will accept: Canadian passport, Enhanced Driver's License/Enhanced Identification Card, NEXUS, FAST/EXPRES and SENTRI enrollment cards.
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https://help.cbp.gov/app/answers/det...-residents-%2F
However I have read many many stories of AIRLINE STAFF not accepting Nexus as valid ID for the flight. They want a valid passport.
So rule of thumb: Bring the passport.