However, the U.S. Passport Card cannot be used for international air travel.
Now keeping your Mid from loosing the thing is a horse of a different color. Example: Lost then found in a coat pocket three months later, lost in a post office in Scotland & missed a train to go back & retrieve it, lost on a ship but popped up in a super secret zippy compartment in a back pack...... I could go on....... Why don't they just micro chip them like they did my doggies?
Make a photocopy of the passport and/or scan it into your computer. That way you will know the passport numbers and get help.
(then beat them!)