Ms Rousseff is accused of illegally manipulating the budget to hide a growing deficit.
She denies the allegations and says the impeachment proceedings amount to a coup d’etat.
Senators are due to vote this week on whether to remove her from office for good or whether to reinstate her.
For her to be removed from the presidency permanently, 54 of the 81 senators would have to vote for her impeachment.
Brazilian daily Folha de Sao Paulo says it has spoken to all the senators ahead of the vote and that 52 have so far declared themselves in favour of the impeachment.
Eighteen told the newspaper they were opposed to the impeachment and 11 either did not say which way they would vote or were undecided.
If Ms Rousseff, 68, is impeached, acting President Michel Temer will serve out her term, which ends in December 2018.
Mr Temer, who was Ms Rousseff’s vice-president, assumed the role of acting president in May when Ms Rousseff was suspended from office pending the impeachment trial.
Ms Rousseff will be given 30 minutes to speak and is expected to give a passionate defence of her time in office.
The suspended president has in the past said that the impeachment proceedings are a ploy by her political rivals to end the 13 years in power of her left-wing Workers Party.
She has argued that moving money from the state bank to fill budget holes is not an impeachable offence and is something her predecessors in office have also done.
After giving her defence, she will be questioned by senators.
The impeachment vote is scheduled for Tuesday but analysts say it could slip into Wednesday.
On Sunday, a few hundred supporters of Ms Rousseff demonstrated in the capital, Brasilia, against her impeachment and called for the removal of Mr Temer.
But in the past months there have also been large rallies against Ms Rousseff and against corruption in politics in general.