My passport will expire soon, but I have a Nexus Card good for a few more years. Is it possible when my passport expires that I can use my Nexus card instead to go into the USA? If so, will there be issues getting back into Canada?
Yes, you can. I use NEXUS (and have a Canadian passport). I renewed my passport a few months ago. I asked border agents from both US and Canadian customs if I was okay during the renewal process, and they advised that I was still allowed to use NEXUS. You may have issues flying, as airlines might insist on you having a valid passport, but land crossings are certainly not a problem.
My next crossing was done under exactly these circumstances and everything went fine.