Funnily enough, a while back (may be different now) I somehow lost my license between the car and the terminal door and didn't have any backup because it was a last minute trip. Guessing it's not the case today post-RealID but I was still able to get through TSA. The kicker though was that I had to go through all sorts of machinations to check into the Travelodge near the airport and pay in cash. Ever since, I use a generally useless global entry ID as identification at the airport and keep my driver's license well secured.