Senators loyal to Brazil's first female president debated through the night in a last-ditch attempt to halt seemingly unstoppable momentum toward her dismissal on charges that she illegally manipulated the national budget.
Despite the impassioned speeches, which followed 14 hours of testimony by 68-year-old Rousseff earlier this week, her fate was apparently sealed.
The Senate judgment session was to start at 11:00 am (1400 GMT) and senators were expected to vote on their verdict a few hours later.
If Rousseff is ejected, her vice president turned bitter political enemy, Michel Temer, will be sworn in at about 3:00 pm (1800 GMT).
The veteran center-right politician, whom Rousseff accuses of using the impeachment process to mount a coup, was to leave by nightfall for a G20 summit in China.
Extra security and the closing of avenues near the Senate in the capital Brasilia caused massive traffic jams. Police said they were preparing for large protests but early Wednesday, the huge esplanade set aside for demonstrators was still eerily empty.