Paul only runs as a Republican to win elections.
His presidential campaign manager has said this.
“Would Paul run as a Republican again or as a Libertarian? "We try not to ever deal in absolutes in politics," Benton said carefully. "But he would be very likely to be running as a Republican again." It's not just that "working within the system" gets more exposure for a candidate. It's that several Republican primary states include the caveat that candidates cannot run in their primaries and go third party if they lose.”
http://reason.com/blog/2008/11/14/ron-paul-2012