Originally Posted by
gj83
bolding mine
Are you referring to TSA? If so, you don't need to show a passport at all to clear TSA, but you are less likely to be given a hard time if you show a US passport. You could show a drivers license or any other documentation on the acceptable list.
I was not aware of any outbound Immigration for the US.
No, outbound immigration, such as it is, is conducted by the airline. It probably doesn't impinge on US citizens at all.
As a non-US citizen I have always simply shown my passport to the TSA people, but it may well be possible to show another form of acceptable photo ID.