Temer has been hanging by a thread since the revelation of a secret audio clip in which he is allegedly heard giving his blessing to payment of hush money by a meatpacking tycoon to a top politician jailed for corruption.
But starting Tuesday, Temer faces the separate, more immediate challenge of the Supreme Electoral Tribunal (TSE).
The TSE is deciding whether abuse of power -- principally the use of corrupt campaign money -- fatally undermined the 2014 election in which Temer was reelected vice president with then president Dilma Rousseff. When Rousseff was impeached last year, Temer took over.
If the TSE judges Temer to have been responsible for abuses in 2014, the court could then annul the election results, throwing Brazil into its second presidential crisis in just over a year.
Street protests
The session starts in the capital Brasilia Tuesday at 2200 GMT. More sessions are scheduled for 1200 GMT Wednesday, then 1200 GMT and 2200 GMT Thursday.
The verdict will be voted on by the seven-judge panel. But the hearing could be adjourned at any point at the request of a single judge, giving Temer some breathing space.
Temer's lawyers said the court on Monday sent them a list of 84 questions which the president had to answer in writing within 24 hours before the start of the first hearing. Temer has not summoned to appear at this week's hearings.
Left-wing groups opposed to Temer and his austerity reforms said they plan protests near the court in Brasilia to demand the president's resignation. Police said they expected several thousand people to join the rallies.
Political pressure on court?
The TSE had previously been considered something of a sideshow. At most, it was expected to put the blame for use of dirty campaign money exclusively on Rousseff.
Since the unpopular leftist leader is already out of the picture -- having been judged guilty of breaking government accounting rules in 2016 -- Temer, a conservative, would be allowed to finish his mandate through 2018.
However the eruption of the hush money allegations against Temer have led to hopes among his opponents that the TSE will seize the opportunity to bring him down.
Raising political tensions, Temer's lawyer Gustavo Guedes on Sunday claimed that Prosecutor General Rodrigo Janot was leaning on the TSE "to find the president guilty."
What could happen?
If Temer is convicted, his grip on power may become untenable.
The election court would decide whether to suspend him immediately or let him remain in office pending a final decision.
He could appeal the verdict, but his center-right alliance in Congress would risk collapse.
Brasilia is swirling with reports meanwhile that Janot plans soon to request that the Supreme Court accept formal corruption and obstruction of justice charges against Temer, triggering a trial.
Janot is also rumored to have more evidence compromising the president, and further suspects in the corruption scandal could strike plea bargains and testify against Temer.
Chief among those is his former aide Rodrigo Rocha Loures, who was caught with a suitcase containing 500,000 reais ($152,000) of alleged bribe money.
In the meantime, the economic austerity reforms that Temer has tried to push through Congress appear increasingly doomed.
Eurasia Group consultants issued a note Monday declaring Temer's chances of survival at 40 percent and the outlook for pension reforms "dire."
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