Former Costa Rica minister Carlos Alvarado Quesada and evangelical Christian Fabricio Alvarado Munoz are tied in the race to be the Central American country's next president, a poll released on Tuesday showed, with four weeks until the runoff vote.

Forty-one percent of respondents plan to pick the center-left Alvarado Quesada in the second-round election, while 39 percent said they would select Alvarado Munoz, a conservative religious singer and former journalist, according to a University of Costa Rica poll.

The margin of error was 3.1 percentage points.

Alvarado Munoz's lead has appeared to shrink in recent weeks, according to other polls.

The 43-year-old Alvarado Munoz, the lone elected lawmaker for the evangelical National Restoration Party, shot to prominence as a candidate after denouncing a court ruling calling on Costa Rica to give civil marriage rights to same-sex couples.

With rival Alvarado Quesada, the ruling-party contender, supporting gay marriage, the April 1 runoff looks set to effectively be a referendum on an issue that has polarized a country known for its laid-back culture and pristine nature.

Alvarado Munoz's rise on a ticket fiercely opposing gay marriage was helped by the decline of a centrist two-party system that stretched back decades in a country long considered one of Latin America's most stable.

Munoz was elected to the national assembly in 2014 as the only federal deputy representing the Christian-backed National Restoration Party (PRN).

One thousand people were interviewed by phone on Feb. 27-28 in the poll, which showed that about 20 percent of Costa Rican voters were still undecided.

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