Same-sex marriage has dominated Costa Rica's presidential election after a human rights court ruled that the nation must recognize such unions.

Voters headed to the polls on Sunday, but it appeared unlikely any candidate will reach the 40 percent threshold needed to avoid a runoff in April.

Conservative Christian evangelical preacher Fabricio Alvarado's poll numbers climbed after he said that he would not support an Inter-American Court of Human Rights (IACHR) ruling calling for marriage equality. As a signatory country, Costa Rica is bound by the ruling.

All of Alvarado's rivals announced their support or willingness to respect the ruling.

Prior to condemning the ruling, Alvarado, who represents the right-wing National Restoration Party, was polling at around 5 percent, but he jumped to first place in the polls to around 17 percent after expressing his opposition to the ruling.

A majority (61%) of Costa Ricans oppose marriage equality, according to a Pew Research Center survey. The nation is predominantly Roman Catholic.