The Supreme Electoral Tribunal (TSE) opened the first of four scheduled sessions to decide whether the 2014 re-election of president Dilma Rousseff and her then-vice president Temer should be invalidated because of corrupt campaign funding.
The seven-judge panel could vote to declare the election result invalid, meaning that Temer -- who took over last year when Rousseff was impeached -- would himself face losing his office.
The conservative president, who faces a separate, potentially devastating corruption probe, says the court will absolve him.
If found guilty at the court's last scheduled hearing on Thursday, Temer would be able to appeal. A judge on the TSE could also decide to adjourn the court hearings, with the whole process potentially still dragging on for some time.
There was a heavy police presence at the election court in the capital Brasilia, with only a small protest by leftwing demonstrators outside.
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