[6] Sánchez would ultimately lead the PSOE into government after a successful vote of no confidence which would see Prime Minister Mariano Rajoy and his People's Party ousted from power.

Susana Díaz became President of the Regional Government of Andalusia and Secretary General of the PSOE–A succeeding José Antonio Griñán—who had resigned as a result of political fallout derived from the ERE scandal—after a rapid ascent among party ranks throughout the previous years.

[12][13] However, the challenge posed by Eduardo Madina—whose pressure forced a primary election to be held on 13 July—,[14][15] Díaz's own desire to become party leader by acclamation and not through a bitter leadership contest which could see her popularity eroded, as well as opposition from within the PSOE–A—members of which asked her to remain in Andalusia—motivated Diaz's decision not to run.

[16][17] Nonetheless, Díaz's opposition to Madina for his maneuvers to trigger a party primary and, consequently, thwarting her planned rise, resulted in the PSOE–A backing a dark horse candidate, then-relatively unknown deputy Pedro Sánchez, ahead of the previous federal congress.

[41] Sánchez refused to step down and instead proposed to have the congress held immediately, scheduling it for 12 and 13 November and the primary election for 23 October, while daring his critics to challenge him in a back-me-or-sack-me vote.

[46][47] In a troubled federal committee held on 1 October, Sánchez finally resigned as party leader after losing a key ballot on his congress' proposal by 132 votes to 107.

[53] Right after government formation, in an exclusive interview for laSexta's Salvados news show, Pedro Sánchez publicly accused Susana Díaz's allies within the party and the "financial powers"—specially the PRISA Group, owner of El País media outlet—of having coerced him into avoiding a left-wing pact with Podemos and nationalist parties throughout the entire government formation process, accusing them of triggering the internal revolt within PSOE to oust him once he considered a serious attempt at forming such an alliance.

[54][55] Taking advantage of the internal disarray resulting from Sánchez's demise, Susana Díaz, main orchestrator of the party revolt that brought him down,[56] became widely regarded as the new PSOE leader in pectore as the Fernández-led caretaker team came to be seen as a mere puppet body under her control.

[62] She also staged a party rally in Jaén on 16 December, in which she was granted the public and unconditional support of former Prime Minister José Luis Rodríguez Zapatero[63][64]—long turned into an admirer of the Andalusian President and an opponent to Pedro Sánchez.

[72][73] Some called for Pedro Sánchez—who had hinted at such a possibility but had never officially confirmed it—to run again for leadership,[74][75] but there were those who regarded both Díaz and Sánchez unfit for the role—arguing that their long-lasting grudge for power had deeply divided the party and that none of them would be able to fully re-unite it again—.

[101][102] Pedro Sánchez, who was said to be privately upset by López's move—both had been close allies during his time as Secretary General—, revealed on 18 January he would be meeting party members throughout Spain without clarifying whether he would withdraw from the race or not.

[103] Moreover, sources from within his team hinted that he had allegedly already chosen to run, but that he would not make the decision public until it better suited him,[104] instead focusing in warming up his potential candidacy with events widely seen as a defiance to his rivals, such as one scheduled in Seville—Díaz's home province—for 28 January.

[113][114][115] While Patxi López had been the first to announce his run and had quickly seen the anti-Díaz coalition coalesce around him, he did not obtain the express support of historical party leaders and long-time friends such as Alfredo Pérez Rubalcaba—who disassociated himself from Lopez's strategy and praised Díaz's "political strength"—.

[119] Sánchez found himself stranded from most of his previous collaborators and could no longer count with the party apparatus of the anti-Díaz regional federations to back him, facing the challenge of making up a minimal infrastructure able to grant his campaign the required resources for remaining competitive.

[131][132] Díaz kept staging various acts as she was allegedly awaiting for the best time to make her announcement, seeking to prevent a possible succession struggle in Andalusia in the event she choose to step down as regional President and leader.

[135] In a speech to a crowd of 5,000 supporters, Susana Díaz formally announced her candidacy on 26 March as planned, supported by former PSOE leaders Felipe González, José Luis Rodríguez Zapatero and Alfredo Pérez Rubalcaba, as well as party figures such as Eduardo Madina, Alfonso Guerra, Carme Chacón, José Bono, 11 former ministers, dozens of local mayors and deputation presidents and most of the regional PSOE premiers, who had already rallied around her throughout the previous months.

[136] Concurrently, Pedro Sánchez increased his campaign agenda in Andalusia—which accounted for about a quarter of the party's membership—aware that Díaz's influence in the region and her expected strong primary showing there could dwarf his support in other communities.

[147] Their personal grudge was so intense that PSOE leaders tried to reduce the campaign tone and cast off fears that the losing side in the primaries might not acknowledge the outcome and split from the party.

[148] Meanwhile, Patxi López presented himself as "the candidate of integration" and the only one able to unite all sides after the primaries—aware that the Díaz–Sánchez polarization was harmful to his campaign—, calling for "unity" to "recover the PSOE" and for his party colleagues to avoid a "major clash" that could result in a permanent rupture.

[154][155] Sánchez, nonetheless, spoke against the organization of the primary process—which he saw as built around Díaz's figure and against his campaign—and warned that "no matter the hindrances and favoritisms, there shall be no wall stopping the force of the Socialist militancy".

[156] Díaz's campaign was shaken in early April by two events: on the one hand, Estela Goikoetxea, curtain raiser in Díaz's 26 March rally, resigned as Director of the Public Health Observatory of Cantabria only three days later after it transpired that she had lied on her curriculum vitae, by assuring she was a graduate in Biotechnology for the University of León despite not actually having finished her studies[157]—with this event also incidentally leading to a crisis in Santander after it was exposed that the local mayor, Gema Igual from the People's Party, was in a similar situation—.

[163][164] The sudden death of former Minister of Defence and party colleague Carme Chacón on 9 April prompted all three candidates for the PSOE leadership to suspend their campaigns for the ensuing days in a show of respect.