In the 2018 presidential election, Arruda Sampaio Jr. entered the race, hoping to be the nominee of the Socialism and Liberty Party (PSOL). Arruda Sampaio Jr. argued that PSOL needed to distance itself from the embattled PT and chart its own course as a party of the left. Labor leader Guilherme Boulos, who eventually received the party's nomination, was seen by Arruda Sampaio Jr. as being too close to former President Luiz Inácio Lula da Silva. At the party convention, Arruda Sampaio Jr. received 27 votes in favor of his nomination, compared to the 87 received by Boulos and the 7 votes received by former CUT official Hamilton Assis.
