Mat Staver, head of the Christian conservative Liberty Counsel, declared last week that signing a pledge to defy a Supreme Court ruling striking down state bans on gay marriage would be a litmus test for all presidential candidates.

Republican presidential hopefuls Rick Santorum and Mike Huckabee have signed the pledge, promising to “stand together in defense of marriage” because allowing nationwide same-sex marriage would “undermine the fundamental rights of children and threatens their security, stability and future.”

Appearing on Newmax's American Forum to discuss the issue, Staver, a co-author of the pledge, said that pledging to defy the possible ruling would be a litmus test for all presidential hopefuls.

“We're going to ask every presidential candidate – Republican and Democrat – to sign on to this pledge and it's going to be very telling if they don't,” Staver said.

However, Hillary Clinton, the only announced Democratic candidate, included an engaged gay couple in her announcement video and has called on the Supreme Court to “come down on the side of same-sex couples being guaranteed” the constitutional right to marry. Other hopefuls, including former Maryland Governor Martin O'Malley and former Rhode Island Governor Lincoln Chafee, are strong supporters of marriage equality.