Transitional President Gen. Mahamat Idriss Deby, whose junta has governed since 2021, had promised to hand power to civilians and hold elections this year before postponing them to 2024.
The "yes" vote is expected to win the constitutional referendum after a well-financed campaign by the ruling junta against a divided opposition, which has faced arrest, intimidation and threats for more than a year.
In the capital, N'Djamena, posters are splashed across walls in support of the proposed constitution, which calls for a "unitary and decentralized state."
Supporters of a federal state urge people in the central African nation and the world's second least developed country, according to the United Nations, to vote "no."
The two main opposition groups have urged a boycott of the vote — some "Halt the Referendum" posters with a big red cross through them have appeared.
But opponents are pinning their hopes on a low turnout, which they say would undermine the transitional president and scion of a family dynasty that has held absolute power in Chad for 33 years.
The proposed constitution is not greatly different from the previous one, which concentrated significant powers in the hands of the head of state.
A unitary state is the sole means of upholding unity, the "yes" camp argues, dismissing federalism as aiding separatism and chaos.
At the age of 37 and then a three-star general, Mahamat Idriss Deby was proclaimed president by a 15-strong junta of generals in April 2021 after the death of his father Idriss Deby Itno.
After ruling with an iron fist for 30 years, Deby died while headed to the front line to lead a battle against rebels in the country's north.
As well as promising civilian rule with elections after an 18-month transition period, Deby junior pledged he would not stand for election.
But 18 months later, Mahamat Deby extended the transitional period by two years and allowed himself to run for president, swapping his army uniform for a traditional boubou or smart suit.
"The result of the referendum is already known. The 'yes' will pass," Badono Daigou, of the GCAP opposition group told a meeting in N'Djamena on Sunday.
'Stay home'
"We call on the people to stay at home on Sunday," Max Loalngar, coordinator of the other main opposition platform, Wakit Tamma, told AFP by telephone.
He was speaking from an undisclosed country where he fled after a bloody repression of a demonstration on October 20, 2022.
After the extension of the transition period, mass protests broke out and between 100 and 300 people were shot dead by police and soldiers, according to the opposition and NGOs.
Local and international NGOs as well as U.N.-mandated experts estimated that more than 1,000 people were jailed before being pardoned. Dozens if not hundreds of them have since disappeared.
Most were supporters of key opposition figure Succes Masra, who returned from exile late October after signing a "reconciliation" accord with Deby.
Masra supports voting "yes" in the referendum whose results are due to be announced on December 28.
Demonstrations have systematically been banned since the October 2022 crackdown.
Legitimacy
"I'm not going to vote because the results are known in advance," said Issa, a civil servant from N'Djamena, declining to give his full name for fear of reprisal.
"Everything is in place for the 'yes' to go through."
But Mahamat Saleh, a self-described economic operator, said he supported the proposed change.
"The unitary state is the choice of the majority of Chadians for preserving unity," he said.
Loalngar, of opposition Wakit Tamma, denounced the referendum as an attempt by the authorities to shore up their hold on power.
"It aims to legitimize purely and simply the dynasty that they would like to impose on us," he said.
Human Rights Watch expressed concern ahead of the vote.
"For this referendum to have any legitimacy, opposition parties and their leaders must feel free to meet and campaign," it said.
"Otherwise, the referendum risks being seen as a means to make the transitional government into a permanent one."
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