[16] In 2012, he earned a PhD degree in Political Science after writing a dissertation titled La lucha por la hegemonía durante el primer gobierno del MAS en Bolivia (2006-2009): un análisis discursivo, dealing with the political discourse during the first mandate in power of Bolivia's Movement for Socialism (MAS), and supervised by Heriberto Cairo Carou [es].

[19] He also collaborated with the GIS XXI Foundation [es] (a polling firm linked to the Venezuelan government) and was appointed as director of its Political Identities research line.

[23][D] He was a member of the editorial board of the political analysis journal Viento Sur [es], linked to IZAN.

[26] Errejón himself became one of the most prominent Podemos politicians in terms of public profile, featuring in TV shows like La Sexta noche.

[26] On 15 November 2014, he was chosen as one of the 11 members of the Council of Coordination of Podemos, commissioned to the post of Secretary of Policy of the party executive board.

[22] Errejón ran as a candidate for Podemos in the December 2015 general election, and was included third in the party list for the Lower House in the constituency of Madrid.

Errejón did not bid for the party leadership (with Iglesias standing as candidate for Secretary-General with the single alternative candidacy of Juan Moreno Yagüe) but presented a project (Recuperar la Ilusión) competing with another two alternatives (around Iglesias and Anticapitalistas, respectively) in the matter of the composition of the State Citizen Council and the voting of several party documents.

[44] Just after Vistalegre II, Errejón was demoted from the position of Spokesperson of the Parliamentary Group in the Congress of Deputies and replaced by Irene Montero.

The crisis' toll took Ramón Espinar (the regional leader of Podemos in Madrid), who quit frontline politics in disagreement with the decision taken by Iglesias of actually presenting an alternative list to Errejón's in the regional election[51] (Podemos had convened not to present an alternative list to Carmena's vis-à-vis the municipal election).

[56] Three days later the platform was launched under the name Más País and Errejón was elected to lead it, vowing "to be at the service of a progressive government".

[61] On 7 January 2020, Errejón congratulated Sánchez and Iglesias during the former's investiture session as prime minister, as both have reached a government coalition agreement, and warned that the action of the new government should not be directed against the opposition right-wing forces in the upcoming legislative term, as it would then buy the right-wing argument of Spain being split in two, rather than what Errejón thought it was actually the case (that "Spain is divided by inequality").

This followed Cristina Fallarás writing about another woman allegedly suffering sexual violence from a "well-known politician who lives in Madrid".

[63] A follower of Ernesto Laclau (whom he has explicitly cited several times as leading intellectual reference)[64] and Chantal Mouffe, Errejón has been considered as the "most forthcoming" figure in Podemos when it came to embrace the party as populist.

When the interviewer pointed out that the population had lost "around 10 kilos on average in recent years", Errejón responded by saying "I don't have that data".