Her government lawyer lodged a last-ditch appeal with the Supreme Court yesterday, but it was unclear whether the court would even respond in time.
Barring a dramatic twist in events, the Senate was to start debating impeachment at approximately 9:00 am, with voting expected either late at night or in the early hours tomorrow.
A majority of more than half of the senators in the 81-member chamber would trigger the opening of a trial and Rousseff's automatic suspension for up to six months. In the final judgement, removing her from office would require a two-thirds majority.
Rousseff says the accounting maneuvers were standard practice for many governments in the past and describes the impeachment as a coup mounted by her vice president, Michel Temer, who will take over if she is suspended.
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A onetime Marxist guerrilla tortured under Brazil's military dictatorship in the 1970s, Rousseff therefore faces possibly her final day in power today.
Journalists gathered outside the gate of the official presidential residence in Brasilia from the early morning hours.
Temer, whose center-right PMDB party broke off its uneasy partnership with Rousseff's leftist Workers' Party, has already prepared a new government, saying his priority will be to take action on the moribund economy, now in its worst recession for decades.
Rousseff vows to resist.
"I am going to fight with all my strength, using all means available," she told a women's forum in Brasilia yesterday.
Rousseff called her opponents "people (who) can't win the presidency through a popular vote" and claimed they had a "project to dismantle" social gains made by millions of poor during 13 years of Workers' Party rule.
However, analysts say Rousseff's fightback probably comes too late and that she had already burned many of her political bridges before the crisis erupted with an awkward style and inability to negotiate.
The country's first female president has also become deeply unpopular with most Brazilians, who blame her for presiding over the recession and a massive corruption scandal centered on the state oil company Petrobras.