You can believe in the concepts and principles of libertarianism without being a Libertarian, just like you can belive in the concepts and principles of democracy without being a Democrat.But can you believe in such principles while being a Democrat? (:<
Seriously, I've often made the distinction between "small l" libertarians (among whom can be included Ronald Reagan, among others including myself), and "big L" Libertarians, the purists that currently dominate the Libertarian Party.
The LP seems to think that their principles can be implemented immediately, while small l's realize that government intervention in our lives is like a dangerous drug from which many addicts will require careful weaning. The LPs are also quite isolationist, while the "small l" crowd is much more likely to support fights around the globe in favor or freedom, or at least against its most bitter enemies.
-Eric