[1][2] The previously dominant National Liberation Party suffered its worst results to date, ending third in the presidential run for the first time in its history with its candidate former Legislative Assembly President Antonio Álvarez Desanti winning only 18%.

Other notable candidates were lawyer and entrepreneur Rodolfo Piza from the center-right Social Christian Unity Party with 16% and right-wing populist Juan Diego Castro with 9%.

[3] The 57 members of the Legislative Assembly are elected using closed list proportional representation through the largest remainder method from seven multi-member constituencies with between four and 19 seats, which are based on the seven provinces.

[7] Carlos Alvarado was the first one of the party to announce his intention to run under the slogan of "I choose the future" (Elijo el futuro) and ran under a progressive platform, which sought to exalt the achievements of Luis Guillermo Solis's government and promised to continue with it.

The Social Christian primary was held on 4 June 2017 with former president of the Health System and previous candidate Rodolfo Piza winning over then deputy Rafael Ortiz.

[10] Former Public Safety Minister Juan Diego Castro launched his unexpected presidential campaign presenting himself as a political outsider and with a strong anti-establishment and anti-corruption speech.

A corruption scandal involving political figures of all the main parties,[11] including candidate Otto Guevara, whose properties and parliamentary office were raided by the General Attorney investigating the case.

[20] Political figures from other parties reacted with outrage including Desanti's wife Nuria Marin and PAC's vice-president nominee Epsy Campbell.

The ruling may require up to 22 signatory countries of the Pact of San José that recognize the contentious authority of the IACHR to change their laws to legalize same-sex marriage (whether by legislative or judicial methods, or both).

Most polls show that most Costa Ricans oppose gay marriage albeit with large numbers supporting it (in a general ratio of 60–40%).

[29] Some analysts point to a possible polarization between conservative and progressive voters, or among religious and secular or older and younger generations depending on the measure, which pushed both Alvarados into the second round.

[30][31] During the run-off campaign, Alvarado Muñoz’ strategy focused on linking his rival with the somewhat unpopular government at the time by pointing out its flaws, errors and scandals.

[33] Muñoz also reinforced his idea of defending "pro-life and family" values, whilst Quesada insisted in wanting an inclusive country with no "second class citizens".

But probably one of the most damaging was the content of a book written by pastor and tele-evangelist Ronny Chaves (close collaborator of Fabricio and described by him as "spiritual father") calling "La Negrita" or Virgen de los Ángeles (Costa Rica's patron saint) a demon.

In the second round Carlos kept San José, Heredia and Cartago Provinces and all of the cantons he won in the first one, and managed to turn Alajuela and Guanacaste on his favor.

[43] The IACHR's ruling on same-sex marriage caused a turmoil in the country, with conservative candidate and staunch opponent Fabricio Alvarado showing a rapid increase in support passing from marginal percentages of around 2–6% to be in the lead with up to 17%.

[46] A post-election study completed by the University of Costa Rica showed that Alvarado Quesada's success was in part due to his performance in the debates, his campaign and defense of the Rule of law.

The last poll of the second round scheduled for 28 March was canceled due to the fact that OPol Consultores' executives alleged that threats had been made against their collaborators.

Catherina Convertino, general manager of Opinión Política CyC, a private limited company that owns both OPol and El Mundo newspaper, confirmed to La Nación that the second round polls had been requested by Juan Carlos Campos, head of the National Restoration campaign and that they decided to make it public following the debt of the party in the total payment of the contract.

However, that changed after a break-up among the National Restoration deputies, which caused more than half to split from the party a few months after the election, thus turning PAC back into first minority.

The left-wing Broad Front suffered a major setback losing most of its seats passing from 9 to 1,[55] whilst right-wing National Integration returns to Congress for the first time in 20 years with 4 seats, but another schism between the party's leadership and the 2018's nominee Juan Diego Castro causes one of PIN's deputies (a Castro loyalist) to separate.