Zuma, 82, is fronting a new opposition party that has become a potential disruptor in the May 29 general election.
But electoral authorities have argued the politician should be barred from the race because of a 2021 contempt of court conviction.
The Constitutional Court in Johannesburg is called to decide on the matter after a lower court sided with Zuma in April. Legal experts say it might take a few days to reach a verdict.
Coming only weeks before what is expected to be the most competitive vote since the advent of democracy in 1994, the case has made some observers nervous.
Zuma's jailing in 2021 triggered a wave of unrest, riots and looting that left more than 350 people dead. There are fears of a repeat.
"Zuma's supporters have threatened violence again this year, should things not go their way," said Zakhele Ndlovu, a politics lecturer at the University of KwaZulu-Natal.
Many in the former president's camp see the Constitutional Court as partisan.
It is the same court that in 2021 sentenced Zuma to 15 months in prison after he refused to testify to a panel investigating financial corruption and cronyism during his presidency. And the head of that panel is now the court's chief justice.
In their filings, Zuma's lawyers argued that he and another five judges who sat on the bench that convicted their client should recuse themselves as "tainted by bias". Were that to happen, the court would not have enough members left to hear the case.
It revolves around the interpretation of a constitutional norm barring anyone sentenced to more than 12 months' imprisonment from serving in parliament. The ban lapses five years after the sentence is completed. The electoral commission (IEC) argued the provision applies to Zuma.
But the political veteran's lawyers successfully contended before the electoral court that it does not, for his sentence did not allow for an appeal and was shortened by a remission.
Ben Winks, a lawyer specializing in constitutional law, said the verdict was "surprising" and the IEC had a strong case on appeal.
"The wording of the constitution... does not talk about how long you served," he said.
Zuma was freed on medical parole just two months into his term.
His new party, uMkhonto we Sizwe (MK), came out of nowhere to become one of the main contenders in the election.
Banking on Zuma's popularity, it is expected to cut into the vote share of the embattled ruling African National Congress (ANC) — the ex-president's former political home.
This could push the ANC towards a return of below 50% for the first time in a national vote. Short of a parliamentary majority, it would be forced to seek coalition partners to remain in power.
The ANC is struggling in opinion polls in the context of a weak economy and allegations of corruption and mismanagement.
An Ipsos poll last month put it at 40.2%, with the liberal Democratic Alliance on 21.9% and the leftist Economic Freedom Fighters 11.5%. MK followed on 8.4%.
Announcing the appeal last month, the IEC said it did not intend to "involve itself in the political field of play" but sought "clarity" to "ensure free and fair elections". In the court filings, it explained that were Zuma to be declared ineligible, this would not affect the election's logistics.
Zuma's name would still appear on the ballot, avoiding a reprint, but he will not be considered as elected after the vote.
South Africans are called to elect a new parliament, which then appoints the president.