The first party using that name was led by Ascensión Esquivel Ibarra as leader and candidate for the 1901 and 1913 elections.

[1] Liberal journalist Otilio Ulate Blanco took control of the party and was part of an oppositional alliance of parties against Republican candidate Rafael Angel Calderón Guardia in 1948 allegedly winning the election.

[2] Ulate's supporters won the war and name him president-elect (even when the military leader José Figueres took temporary control of the government with a de facto Junta) and a Constituent Assembly was called to draft a new Constitution, with PUN winning most of the seats.

Nevertheless, the following election Ulate's PUN and Calderón's PRN went separate postulating each of the parties’ leaders, splitting the opposition's vote and with PLN gaining triumph.

PUN and PRN agree to stand together in the 1966 election with a consensus candidate; college professor José Joaquín Trejos earning a very tight victory.