Originally Posted by
catocony
The OP will only go through TSA once, and that's on the return from Brasil. After clearing Immigration and Customs in ATL, he'll have to go through TSA to get to the flight to YYZ. He doesn't need a passport, I would hope that TSA can figure out a Canadian driver's license. If not, either passport will do, as it's not important beyond matching a boarding pass to some form of photo ID for TSA purposes.
I don't know if a Canadian DL is acceptable to the TSA. I can tell you that an Israeli one, which is okay with any American traffic cop, is not with the TSA.
I show my US passport to the TSA instead of my Israeli one because as an American citizen I don't have a visa in the Israeli passport.