Three judges in Brazil will shortly decide whether to uphold a prison term given to the country's former President, Luiz Inacio Lula da Silva.
The ruling could prove vital to Brazil's political future.
Despite being convicted of corruption and money-laundering in July 2017, Lula is hoping to win another term as president at elections on 7 October.
He firmly denies the allegations against him, which he says are designed to stop him running for office again.
So it's a choice of prison or the presidency?
Lula, 72, has been sentenced to nine-and-a-half years' imprisonment, but if the judges don't overturn that he won't be sent straight to jail.
He has remained free while the most recent appeal was heard, and could still appeal to higher courts.
If the sentence is upheld it would likely put paid to his re-election ambitions, though - angering his many supporters.
What's happening on the streets?
Thousands of Lula's supporters have been rallying in the southern city of Porto Alegre, where the appeal has been heard.
Addressing the crowd on Tuesday, the Workers' Party figurehead said only death would stop him persevering.
"Only one thing will take me off the streets of this country, and it will be the day of my death," he declared.
"Until that moment, I will fight for a more just society. Whatever the outcome of the trial, I will continue fighting for the dignity of the people of this country."
Anti-Lula demonstrators took part in a rival march in Porto Alegre, demanding an end to corruption.
What exactly is Lula accused of?
When left-winger Lula rose to power in 2003, he promised an end to shady, corruption-ridden politics. Then in 2005 a huge vote-buying scandal nearly cost him his job.
Despite that, he won the support of the poor by pouring billions of dollars into social programmes, and left office in 2011 with record approval ratings.
It was Brazil's biggest-ever corruption scandal, Operation Car Wash, which triggered Lula's current legal woes.
The investigation, which began in March 2014, sucked in more than 80 politicians and members of the business elite.
In 2017 Lula was found guilty of accepting a beachfront apartment from an engineering firm in return for help in winning contracts for Petrobras - Brazil's state oil company.
He also faces other charges of money laundering, influence peddling and obstruction of justice. He has repeatedly denied those claims.
What happens next?
Lula, a former steel worker turned union boss, came to office as Brazil's first left-wing leader in almost half a century. He remains one of the best-loved politicians in Brazilian history.
But critics insist he is a hypocrite - and a symbol of endemic political corruption.
If he does get to run in October, opinion polls give him a good chance - his support is currently twice that of his nearest rival.
Whatever the judges in Regional Federal Tribunal 4 decide, it could well trigger protests from pro- or anti-Lula factions in the days to come.
A security operation is under way in Porto Alegre to prevent clashes between the two sides.
Helicopters will fly over the court building, while ships are being deployed to protect it on a nearby river.
Lula will not be present in court, and is expected to follow the outcome from his home in Sao Paulo.