As has been discussed on this site in the past, technically speaking, you do not need any identification at all when traveling domestically in the United States. If you forget your I.D., when checking in, politely tell the airline check-in staff and they will mark your boarding pass for secondary screening. TSA
will then subject you to secondary screening. Now then, TSA
could deny someone boarding if it wanted to, but personal experiences posted by people on this site indicate that if you are polite and respectful, 99 percent of the time TSA
will let someone without identification board after being subjected to secondary screening. This happened to my mother-in-law: She forgot her I.D., informed the airline check-in staff of that fact, the airline marked her boarding card, and TSA
subjected her to secondary screening and allowed her to board. Again, TSA could
deny boarding, and it is always a good idea to travel with government-issued photo I.D., but technically speaking it is not required.