I had never really given it much thought, until I read this article. Politico claims that there are five viable options:
1. Keep it
2. Spend it
3. Donate it
4. Convert it for other political uses (like Rick Perry)
5. Do nothing
My best guess is that Santorum will keep it, in anticipation of another run for office. If I were him, however, I would try to find a loophole... use the campaign money to start a sweater-vest company!
Check out the attached Politico article for more.