You shouldn't need a passport.
The US changed the rules a year or so ago... probably to get back at our border control folks.
Americans didn't need a passport to come to Canada, but you had to prove you were American and some of the border control folks were apparently insisting that a passport was the only acceptable proof.
Once you showed the passport and proved you were American then of course the passport was no longer required and you were free to enter with any
other form of suitable identification.
A few years ago I did manage to convince one of my friends that an NRA membership card was required in order to cross the border at Detroit/Windsor.
