Former Pennsylvania Senator Rick Santorum last week became the first presidential candidate to sign a promise to oppose marriage equality as president.

The National Organization for Marriage (NOM) announced that Santorum signed the pledge during its Second Annual Marriage Gala in Washington, D.C. on July 2.

(Related: NOM asks presidential candidates to pledge opposition to gay marriage.)

According to NOM, Santorum brought the crowd of nearly 400 people to their feet with his promise to work to reverse the ruling as president, saying, “This will not stand.”

Santorum is not alone in denouncing the ruling. He is joined by all of his Republican rivals.

While Wisconsin Governor Scott Walker and Texas Senator Ted Cruz have endorsed a constitutional amendment defining marriage as a heterosexual union in response to the ruling, others, including Santorum, have vowed to defy the decision.

(Related: Rick Santorum: Gay marriage ruling vindicates “man on dog” warning.)