Johnson, the former Republican governor of New Mexico, dropped his bid for the Republican presidential nomination late last year in favor of running as a Libertarian. Prior to that point, Johnson’s supporters protested that he was unfairly excluded from GOP debates. He participated in one GOP debate, in September, when he memorably joked, "My next-door neighbor's two dogs have created more shovel-ready jobs than this current administration."

Johnson’s disavowal of the likely Republican choice for president is a demonstration of both his dissatisfaction with the party and the differences between Romney’s and his platforms. Johnson supports legalizing marijuana and ending the war on drugs, and does not lean as socially conservative as most of the Republican field. He has the most in common with Rep. Ron Paul (R-Texas), who is still contending for the Republican nomination and has run in the past as the Libertarian Party nominee.

Johnson officially accepted the nomination from the Libertarian Party at the national convention in Las Vegas last weekend, making him a potential third-party spoiler in November.