Brazilian President Dilma Rousseff reacts during a Summit of Heads of State of MERCOSUR and Associated States and Meeting of the Common Market Council in Brasilia in this July 17, 2015 file photo Thomson Reuters Huge crowds on Sunday are expected to demand Brazilian President Dilma Rousseff's impeachment, injecting more volatility into an increasingly toxic political struggle in Latin America's biggest country.

Rousseff, less than a year into her second term, is on the ropes as the world's seventh-largest economy and host of the 2016 Rio Olympics slides into recession.

Austerity measures have replaced the economic go-go years fueled by Chinese demand for commodities, while a bribes and embezzlement scandal spreading ever deeper through the ranks of the country's elite has tainted Rousseff and her leftist Workers' Party, even if she herself has not been directly implicated.

Now demonstrators in Rio de Janeiro, the capital Brasilia, the financial powerhouse Sao Paulo and scores of other cities are expected to back calls in Congress for Dilma, as the president is commonly known, to be impeached.

"Everyone on the street!... Time for impeachment!" Revoltados ON LINE, one of the three groups organizing the "mega-demonstration," declared on its Facebook page.

At least 600,000 people turned out against Rousseff in April and more than a million in March. Since then, the mood -- like the economy -- has only darkened.

A key figure in the fragile governing coalition, House Speaker Eduardo Cunha, defected in July and is considering whether to pull the trigger on calls in Congress for Rousseff's impeachment.

While Cunha has not openly called for Rousseff's departure, some see him as the eminence grise in a campaign to bring her down. And even if impeachment proceedings never start, the threat has added to an atmosphere of crisis.

Rousseff, who was tortured in the 1970s when she was a leftist guerrilla during Brazil's military dictatorship, likens impeachment talk to a "coup d'etat."

"The political environment has really deteriorated," said Gabriel Petrus, a political consultant at Barral M Jorge Associates in Brasilia.

Protestors in Brazil. Mario Tama/Getty Images

Corruption black hole

At the rotten heart of Brazilian politics is the mammoth bribes and embezzlement scandal centered on state-owned oil giant Petrobras.

Crusading prosecutors have brought charges against high-ranking executives and politicians, shaking Brazil's establishment and feeding growing public ire.

The roll call of suspects is a Who's Who of Brazilian movers and shakers, including the billionaire head of the global construction company Odebrecht and a navy admiral once tasked with overseeing a secret nuclear program.

Even Rousseff's presidential predecessor, Workers' Party hero Luiz Inacio Lula da Silva, is being investigated in an unrelated influence peddling probe. A former top aide of Lula's was arrested this month on accusations that he was a chief instigator in the Petrobras scheme.

Rousseff herself has not been accused but she chaired the board at Petrobras between 2003-2010, when much of the alleged corruption was flourishing. The treasurer of the Workers' Party was among those arrested in April.

Not that her enemies have anything to celebrate: Cunha himself is alleged to have demanded a $5 million bribe in the Petrobras corruption network.

Rousseff waves during a meeting with governors at Alvorada Palace in Brasilia Thomson Reuters

Backroom deal?

Against this ugly backdrop, Sunday's rallies will take on a more overtly political tone.

For the first time in a series of street protests, the business-friendly opposition Brazilian Social Democratic Party (PSDB) has endorsed the march. PSDB leader Aecio Neves, whom Rousseff narrowly defeated in 2014, will be there himself.

But behind the scenes, Rousseff is looking for a way out.

Earlier this week, she and Senate President Renan Calheiros, who is also being investigated in the Petrobras affair, agreed to a market-pleasing package of reforms called the Brazil Agenda.

The deal, which takes Rousseff ever further from her socialist roots, could help lure her right-wing opponents from the cliff's edge by giving them a plausible reason to cooperate.

The problem for Rousseff is that while she may escape her opponents, her rock-bottom approval ratings show she has lost the base that once accepted her as Lula's chosen successor.

Petrus, who knows Rousseff personally, says Brazil is not about to turn into a basket case like he feels Venezuela is.

"It's a democracy," he said.

But that doesn't mean things are going to get easy for Rousseff.

"She doesn't like politics," he said. "She's not a politician. That may explain why she looks so sad."