Seven years after taking office by ousting corruption-ridden conservatives from government, Pedro Sánchez is fighting for his political life amid investigations into alleged graft in his Socialist party (PSOE).

On June 12, an ashen-faced prime minister apologised to Spaniards after audio gathered by civil guard investigators was made public and appeared to show the PSOE secretary, Santos Cerdán, discussing commissions paid by companies in exchange for public contracts.

Sánchez has not himself been directly implicated, but the Socialist leader who came to power promising to clean up politics is now facing calls to resign from an invigorated opposition.

Cerdán, who was party number three, has resigned from the PSOE and stepped down as a member of parliament. He is due to appear before the Supreme Court on 25 June. He maintains he has never committed a crime nor been implicit in one.

The investigation into commissions is part of an ongoing probe which has already implicated José Luis Ábalos, a former PSOE secretary and transport minister. A third person implicated is Koldo García, an advisor to Ábalos. Both men featured with Cerdán in the recently exposed audio. All three say they have done nothing wrong.

The investigation into Ábalos, which began last year, was damaging for the government but his exit from the cabinet and the PSOE secretary post in 2021 put distance between him and Sánchez. However, the implication of Cerdán is more problematic.

Sánchez had repeatedly defended him in the face of claims in the right-wing media over recent months that he was under investigation, and the prime minister even accused the opposition of "slandering honest people" when asked about Cerdán's activities last month.

The party secretary, from the northern region of Navarre, was a trusted confidant of the prime minister, playing a crucial role, for example, in negotiating the support of Catalan nationalists to allow the formation of a new government in 2023.

Despite acknowledging that he "should not have trusted" Cerdán, Sánchez has insisted that he will see out the legislature, which is due to end in 2027.

In a letter to PSOE members he apologised again, while doubling down.

"There are many issues that affect the lives of the majority – healthcare, housing, pensions, jobs, fighting climate change and defending equality – and for which it is worth fighting still," he wrote. "Challenges that are not solved with headlines or lynchings."

However, the opposition has presented the investigation as symptomatic of a corrupt regime, pointing to other probes affecting Sánchez and his circle.

A judge has been investigating the prime minister's wife, Begoña Gómez, for possible business irregularities - and his musician brother, David, is due to go on trial for alleged influence peddling in taking up a public post in the south-western city of Badajoz. Meanwhile, the Attorney General, Álvaro García Ortiz, is also likely to face trial for revealing confidential details of a tax evader. All three deny wrongdoing.

Sánchez and his supporters have cast these three affairs as part of a campaign orchestrated by the conservative People's Party (PP), the far-right Vox, right-wing media and factions within the judiciary. A number of judicial experts have expressed surprise at the zeal with which the investigations have been carried out.

In a raucous parliamentary session this week, opposition MPs chanted "Dimisión" (Resign) at the prime minister, and Alberto Núñez Feijóo, leader of the PP, accused him of being "a wolf who has led a corrupt pack".

Paco Camas, head of public opinion in Spain for polling firm Ipsos, sees a Sánchez resignation as "political suicide" for his party, because it would almost certainly trigger elections, allowing the PP to form a government, probably with the support of Vox.

"The overall trend right now is a demobilised electorate on the left, particularly for the Socialist party, and an enormous mobilisation of voters on the right, which is capitalising on the discontent with the government," Camas said.

Even the Socialist president of the Castilla-La Mancha region, Emiliano García-Page, has warned that "there is no dignified way out" for the PSOE.

However, as long as Sánchez can keep his fragile parliamentary majority of left-wing and nationalist parties together there is little the opposition can do to bring him down.

To that end, the prime minister has been frantically trying to reassure these allies, many of who have voiced outrage at the Cerdán-Ábalos scandal. Camas believes that persuading them to support a 2026 budget could be a way for Sánchez to buy some time.

Nonetheless, such plans could be left in tatters were more explosive revelations to emerge, as many in the Socialist party fear.

Such worries will be playing on Sánchez's mind as he heads to the Nato summit in The Hague.

Normally an assured presence on the international stage, he will arrive with serious doubts about his future and under mounting pressure to raise Spain's defence spending.

Although his government has promised to increase military spending to 2% of economic output this year, it has been resisting calls from the United States and the Nato leadership to raise it further. Sánchez has now refused to accept a target of 5% of GDP for military spending, saying it "would not only be unreasonable but also counterproductive".