When flying internationally, can a TSO demand that you show your passport as ID instead of, say, a driver's license? Or would a driver's license suffice when presenting boarding pass and ID, even if flying internationally?

It is not a requirement that one has to present a U.S. Passport when traveling internationally, however there is nothing to say that a TSO can't request to see another form of ID such as a Passport either. Generally, the Passport is the most often used form of ID by those traveling internationally.

Having said that, please be aware that airlines are responsible for verifying that a passenger carries a valid Passport for international travel and most airlines will either verify prior to issuing a boarding pass, or when presenting the boarding pass at the boarding gate.