A debate over gay marriage has upturned Costa Rica’s presidential race, giving ammunition to conservative frontrunners ahead of Sunday’s vote and challenging the Central American country’s image as a progressive bastion. Evangelical Christian singer and congressman Fabricio Alvarado, who leads recent polls, has pitted himself against a January ruling from the Inter-American Court of Human Rights that urges Costa Rica to legalize same-sex marriage. Alvarado, 43, has said the Costa Rica-based court is violating the country’s sovereignty and jeopardizing its “traditional family” values. He has threatened to withdraw Costa Rica from the court system, as Venezuela did.