Will I be denied entry into New Brunswick from Maine if I attempt to cross with an expired passport? What if I had my Social Security card, birth certificate, driver's license, and W-2? Would that documentation be enough for entry into Canada and re-entry into the U.S.?
You should expect to be detained at the border and have your vehicle searched in both directions if you travel without a valid passport. Coming back into the US, you may even be invited to have a seat in a holding cell. This may add minutes or possibly even hours to your journey.
In both directions you are eventually going to be admitted. Canada still admits Americans with a birth certificate and driver license, and the US will eventually confirm your citizenship and release you.
If time permits (i.e. you have at least 24 hours before your trip and can get to Portsmouth, NH) you should get your passport renewed in order to avoid such inconveniences.
I once arrived at the border headed to Canada only to realize that one of the passengers in my vehicle could not locate their passport. Remember, when entering Canada, you're speaking to a Canadian, when entering the US, you're speaking to a US CPB agent.
After explaining the situation, the Canadian officer said "I trust that you are entering the country for the reasons you explained and will allow you to enter. When you attempt to re-enter the States, you will have to do the same thing to the person you speak to.
The trip was over a weekend so we were not able to contact any government agency although we were able to get a photo copy of an expired passport.
The person was supposed to fly back to the US and the airlines agent explained that they (the airline) are not permitted to allow anyone through security on an international flight without a valid passport so she would not be permitted to board the plane HOWEVER, the US CBP agent does have the latitude to allow you in without the actual passport, you just need to find a way to speak to them (not by air).
Her suggestion was to board a bus from Vancouver to Seattle and in that scenario, you speak face to face with the CPB officer and "plead your case".
Long story short, it worked and all was fine but it was a stressful weekend "vacation" :) All of this is to say, I think you should be fine with all of that documentation but don't be surprised if you get a little more scrutiny. Good Luck!