Temer, who also faces a growing, separate corruption scandal, says he is confident the court will absolve him and allow him to serve out his term to the end of 2018. However, the seven judges on the Supreme Electoral Tribunal (TSE) could trigger a political earthquake.
At issue are allegations that the 2014 re-election of president Dilma Rousseff and her then-vice president Temer was fundamentally flawed because corrupt campaign funds were used.
If the court votes to scrap the election result, Temer -- who took over only last year when Rousseff was impeached -- could lose his job, forcing Brazil's congress to pick an interim president.
The upheaval comes just as Brazil has shown the first glimmers of exiting its worst recession in history.
The first judge scheduled to cast a vote was the coordinator of the case, Herman Benjamin. He is widely expected to rule against Temer.
However, the president of the TSE, Gilmar Mendes, has indicated he will vote for tossing out the charges.
Warning against "exaggerations," he said the court should not undermine the role of elections, or the "value of the popular will, whether it was right or wrong."
The court has been holding sometimes heated deliberations on the upcoming verdict since Tuesday, with sessions scheduled throughout Thursday and if necessary through Friday and Saturday.
Double trouble
If found guilty at the TSE, Temer would be able to appeal. But analysts say that the initial reading of the court's deliberations makes acquittal more likely.
jpegMpeg4-1280x720"There are growing signs President Michel Temer could well enjoy a majority on the court," Eurasia Group consultancy said.
Whatever the result of the TSE verdict, Temer faces an equally serious threat from a probe into alleged obstruction of justice.
Prosecutor General Rodrigo Janot accuses Temer of having agreed to pay hush money to former lower house of Congress speaker Eduardo Cunha, who is in prison for corruption. Temer says the central piece of evidence -- a secretly made audio recording of a conversation he had with a meatpacking industry tycoon -- was doctored.
Analysts say that if the TSE absolves Temer, Janot could accelerate his own legal assault and present formal charges.
In that case, the lower house of Congress would have to approve the charges by a two thirds majority, likely forcing Temer's ouster. However, the process could be lengthy and Temer is working daily to maintain enough support among legislators to defeat the charges.
If Temer is forced from office, the speaker of the lower house would take over for 30 days during which legislators would choose a new interim president to serve through 2018. The lack of a clear consensus figure is thought to be the major reason why allies have not yet deserted Temer.
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