One intervention that received significant attention was by Pope Francis, who addressed Slovenian pilgrims during a general audience at St. Peter Square in the Vatican on Wednesday.

He told them to "communicate my gratitude to the entire Church in Slovenia for its efforts for the benefit of the family. I wish to encourage all Slovenians, especially those in public capacity, to preserve the family as the basic unit of society."

A similar message was delivered yesterday by Jan Figel, former European commissioner for education who is now the deputy speaker of the Slovakian parliament.

He told a debate hosted by two conservative institutes that family ought to be protected, which Slovakia did with constitutional amendments defining marriage as a union between man and woman.

The yes camp, meanwhile, received an endorsement Thursday from Human Rights Watch, an American NGO, which joined a large chorus of Slovenian human rights NGOs calling for marriage equality.

"The rights of a minority, particularly to equality, shouldn't be subject to removal by the whim of the majority," said Boris Dittrich, lesbian, gay, bisexual, and transgender (LGBT) rights advocacy director at Human Rights Watch.

"The right to marry is a fundamental right, as is the right not to be discriminated against, and same-sex couples should not be denied the right to marriage equality."

A surprise endorsement also came from Jesse Tyler Ferguson, best known for playing the Mitchell Pritchett on the sitcom Modern Family and the co-founder of Tie The Knot, a LGBT advocacy group.

"Myself & everyone at @tietheknotorg believe in equality for ALL families. Good luck to the Slovenian "yes" campaign on Sunday!," he wrote on Twitter.