Brazil's Senate on Tuesday debated before voting on whether to send suspended president Dilma Rousseff to an impeachment trial, bringing the Olympic host country's political crisis to a climax.

At the start of the marathon session, Supreme Court President Ricardo Lewandowski reminded senators that they were about to "exercise one of the most serious tasks under the constitution."

The Senate speaker, Renan Calheiros, also underlined the seriousness of the impeachment process, which Rousseff, a one-time Marxist guerrilla, has likened to a coup d'etat.

"I want to emphasize the gravity of the decision that we will soon take," he said. "I ask that we set aside as much as possible our party political convictions."

Rousseff's opponents only need a simple majority of the 81 senators' votes. They look set to clear the threshold easily, although the debate is expected to stretch into the early hours of Wednesday.

It is the final vote before the one that will decide Rousseff's ultimate political fate, when a two-thirds majority would be needed to strip her of her power and end 13 years of leftist rule in Brazil.

That vote is expected to take place around the end of August, just days after the Rio Olympics end.

The Senate suspended Rousseff, Brazil's first woman president, on May 12 over accusations of illegal accounting practices and fiddling the budget to mask a slumping economy.

The timing could hardly be more awkward for Brazil, which was meant to be showcasing its burgeoning economic clout and political stability with South America's first Olympics.

Instead, unpopular interim president Michel Temer -- formerly the vice president -- is struggling to drag the country out of its worst recession in decades as the Senate debates what to do with his former boss and bitter enemy.

Rousseff's allies in the Workers' Party point out that many of the lawmakers accusing her are implicated in corruption cases arguably far more serious than accounting tricks.

But her enemies say her fate is already sealed.

"The president is ever more isolated, a very pronounced isolation that has only gotten worse in recent weeks and now even includes her own party," said Senator Aloysio Nunes of the opposition party PSDB.

"I have no doubt that the vote will be in favor of impeachment, as it will be at the final trial," he told AFP ahead of the Senate session.

- Messy end game -

The impeachment trial is set to open around August 25 -- four days after the Olympics closing ceremony -- and last five days, concluding with a judgment vote.

The political mess makes an unfortunate backdrop to the Olympics, awarded to Rio de Janeiro in 2009 when Rousseff's predecessor and political mentor Luiz Inacio Lula da Silva was hugely popular.

Now, not only is Rousseff facing political oblivion, but Lula is set to go on trial on obstruction of justice charges. He also faces corruption allegations that could sink his hopes of returning to power.

Rousseff, 68, was jailed and tortured by the country's military regime in the 1970s when she belonged to an urban guerrilla group.

She rode Lula's coattails to power when term limits forced him to step down in 2011.

But the buoyant national mood soon deflated as Brazil's booming economy sank into its worst recession in 80 years and a huge corruption scandal erupted at state oil giant Petrobras.

Rousseff is not facing corruption charges in the wide-ranging scandal. But she has been tainted by its stain on the Workers' Party, which is accused of lining its coffers with some of the missing billions.

If Rousseff is removed from office, Temer, her center-right running mate turned opponent, will become the full-fledged president until the next elections in 2018.

He has urged the Senate to move quickly, saying "people need to know who the president is."

Disgust with the entire political class is widespread in Brazil.

Temer, 75, presided over the Olympics opening ceremony Friday, drawing boos from the crowd -- just as Rousseff did at the opener of the 2014 World Cup.

Controversially, several people have been expelled from Olympic events for holding up signs calling for Temer's resignation.

A judge ruled Tuesday that peaceful protests must be allowed in the stadiums.

Rio's Olympic organizing committee said it would appeal.

Source: AFP