Alberto Fernández’s popularity has suffered during the Covid-19 pandemic, as illustrated by footage, replayed repeatedly on local television, of Argentina’s president speeding away to safety from stone-throwing protesters during a visit to Patagonia.

But observers say the greatest threat to his hold on power comes from just one person: his vice-president, Cristina Fernández de Kirchner.

That has been a constant concern since she surprised the nation by announcing that her one-time cabinet chief would run for president in her place in the 2019 elections. But the so-called “VIP vaccines” scandal that erupted a month ago has crystallised the threat.

The affair, which exposed queue-jumping for Covid-19 jabs and led Fernández to axe a close ally, health minister Ginés González García, has forced the president to turn to his administration’s core supporters, who regard Fernández de Kirchner as their true leader.

“We no longer have any doubt as to who is in control; it is Cristina Fernández de Kirchner. Uncertainty is never good for investors, but neither is this realisation,” said Jimena Blanco, an analyst at Verisk Maplecroft, a risk consultancy, of the Peronist politician who clashed with investors during her two-term presidency in 2007-2015.

One of the clearest signs that Fernández has bowed to his vice-president was his annual address to congress on March 1, which represented a turn away from the more conciliatory speech he gave a year ago. Instead, Fernández rekindled conflicts with the perennial enemies of the “kirchneristas”, including the judiciary.

Three days later, Fernández de Kirchner lashed out at Argentina’s justice system and “lawfare”, or the use of the courts to attack political enemies, when she gave testimony in one of the nine corruption cases she faces. She accused judges of persecuting her, being “rotten and perverse”, and of systematic political interference, as she angrily jabbed her index finger at the camera.

The attack was given extra force because it coincided with a Brazilian court ruling that annulled corruption charges against former president Luiz Inácio Lula da Silva over a technicality.

Roberto Saba, an Argentine lawyer, said that Fernández de Kirchner is using the weaker cases against her, the so-called “future dollars” case that accuses her government of defrauding the central bank, to attack the courts and elites. But the notion of lawfare “erodes the legitimacy of courts, which is extremely dangerous”, he warned.

Fernández de Kirchner’s crusade against the judiciary intensified after Lázaro Báez, a close associate of the Kirchner family, was sentenced to 12 years in prison for laundering $55m of dirty money. This has direct implications for Fernández de Kirchner, as the money is alleged to have originated from corrupt schemes with the Kirchner family, claims that are being determined in a parallel case in which she is the main defendant. She denies all charges.

Her fresh offensive triggered the resignation of another of the president’s most loyal aides, the moderate justice minister Marcela Losardo. The announcement last week of Losardo’s replacement with Martín Soria, a well-known critic of the judiciary, has been interpreted as yet another win for Fernández de Kirchner, the latest in a string of recently-appointed “kirchneristas”.

The president insists nothing has changed in his relationship with his deputy. “I may have differences with Cristina . . . But I arrived with Cristina, and I will leave with Cristina [too],” he said in a recent interview.

But Graciela Römer, a political analyst, said that the importance of winning the approaching midterm legislative elections in October has put the government “in a very delicate situation”.

An attempt to keep supporters onside also explains why the government looks likely to postpone the renegotiation of a $44bn loan from the IMF granted in 2018, she said, as a deal with the Washington institution could anger the “kirchneristas”.

“It’s increasingly clear that the person who is leading the campaign right now — if not the government itself — is Cristina Fernández de Kirchner,” said Römer.