Former Pennsylvania Senator Rick Santorum on Monday formally declared for the 2012 GOP presidential nomination.

Santorum is the eighth candidate to enter the increasingly crowded race. On Thursday, former Massachusetts Governor Mitt Romney threw his hat in the ring.

One candidate – former New Mexico Governor Gary Johnson – backs government recognition of gay and lesbian couples with civil unions, and an openly gay candidate, Fred Karger, supports gay marriage.

While the other five candidates – Romney, former Minnesota Governor Tim Pawlenty, former House Speaker Newt Gingrich, former Godfather's Pizza CEO Herman Cain, and Texas Representative Ron Paul – oppose gay rights, Santorum's comments are considered among the most offensive.

In 2003, Santorum created a firestorm of controversy that has yet to quell when he likened gay unions to “man on dog.” In January, he said opposition to gay rights is “common sense.” More recently, he's gone on the defensive, denying that he's homophobic and insisting that he has friends who are gay.

When Good Morning America host George Stephanopolous asked Santorum if he had a credible path to the nomination, Santorum responded that voters will back him because he's stuck to his core conservative principles.