Fachin, who was named to the top court by Rousseff in June, also argued before his fellow justices that the Senate does not have any authority to review the grounds for impeachment once the lower house votes to accept the charges by two-thirds of its members.
The full court will vote on the issue on Thursday afternoon. If it adopts his view, the Senate will have to open an impeachment trial right after the house approves the charges. A move to trial would suspend Rousseff for up to six months with Vice President Michel Temer taking over as Senators debate to remove her permanently or clear her, returning her to office.
"The house decides on the admissibility of impeachment, the Senate judges ... it must necessarily begin a trial," Fachin told the 11-member court, rejecting most of the complaints lodged against the impeachment case by the Communist Party of Brazil, a small ruling coalition ally.
The impeachment proceedings, started this month by Rousseff`s archenemy, lower house Speaker Eduardo Cunha, risk plunging Brazil deeper into political turmoil as the government struggles to tackle the deepest recession since the early 1990s and dozens of politicians, among them Cunha, are the targets of a massive corruption probe.
Late on Wednesday, federal prosecutors, who have charged Cunha with taking bribes, asked the Supreme Court to order him to step down from his position as speaker.
The impeachment request is based on the allegation that Rousseff used accounting tricks to ramp-up spending beyond legal limits during her re-election campaign last year. She says she committed no crime and the bid to oust her is a "coup" against her democratic election.
"They will not achieve anything by attacking my record, which is known; I love my country and I`m honest," she told a convention of youth groups late Wednesday in Brasilia. "I will fight against the illegitimate interruption of my mandate using all the tools that the rule of law gives me."
The secret ballot in the lower house, convened by Cunha, allowed members of Rousseff`s fragile ruling coalition to break ranks and vote against the unpopular president. Opponents of Rousseff were able to approve an impeachment committee stacked with legislators hostile to her.
However, Fachin stepped in to prevent the impeachment committee from meeting, pending a Supreme Court ruling.
If the house committee gives the green light to impeachment proceedings, two-thirds of the entire lower house must vote to move ahead with them. Rousseff would then be tried before the Senate, which would also require a two-thirds majority to remove her from office.
Rousseff is believed to have enough votes currently to block impeachment in the lower chamber - 171 of the 513 seats - though a growing rift with her main coalition ally, the fractious Brazilian Democratic Movement Party, threatens to reduce her narrow margin.
Rousseff`s position is stronger in the Senate, her last line of defense, but if the impeachment case drags on into next year public pressure on the Senate to remove her could grow due to the recession that is fueling inflation and unemployment.
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