SuperCargo

Yes, ignorant agents. Airline or immigration agents unaware that Canadians can travel with just a NEXUS card to the US.

I'm confused what we're arguing over here. You yourself have included documents in your reply saying that Canadians only need a NEXUS card to enter the US. Can you come up with a single situation, given the documents cited by myself and others, where a Canadian holding a NEXUS card would require a passport?

Again, I refer to the CBP:
https://help.cbp.gov/app/answers/det...al-information

This includes this paragraph: "All Canadian Citizens entering the United States by AIR are required to present a valid passport, or a NEXUS card when departing from Canada. Please be advised that if arriving in the United States on a flight not pre-cleared by the CBP in Canada, Global entry kiosks cannot be used by passengers only holding a NEXUS card, and the regular inspection lanes have to be used." That paragraph is entirely clear: if you are Canadian travelling to the US on a non-pre-cleared flight with just a NEXUS card, you have to get into a regular inspection queue. Had I found this page sooner, I could have saved myself some time going through US Federal Register pages (not fun!).

So in summary, based on information detailed on CBP sites:
- Canadians can enter the US solely with a NEXUS card by land or sea when travelling within the Western Hemisphere. The CBP goes as far as to say that Canadians can enter the US from Mexico using their NEXUS cards in the SENTRI lanes, if they register their vehicle -- but the WHTI makes clear that they can use their NEXUS cards for entry in a regular lane from Mexico if their vehicle is not registered.
- Canadians can enter the US solely with a NEXUS card when travelling by air into the US into Canada, regardless of whether they are on a pre-cleared flight or not. On a non-pre-cleared flight, you are supposed to get into a regular immigration queue if you only have a NEXUS card on you.

It is unclear what the rules are if a Canadian tries to enter the US by air from a third country, but frankly that's not a realistic situation anyways, as Canadians would need a passport to enter the third country to begin with.

I originally thought you were arguing that Canadians need a passport if travelling to the US by air if not using a Global Entry kiosk at the departure airport. Now I have no idea what you're arguing! One is never guaranteed entry in another country unless you're a citizen, but as a Canadian NEXUS cardholder, not carrying a passport shouldn't be a reason to be denied entry to the US.