The copper-rich country is sharply divided, with polls indicating that the new text will get rejected, despite huge popular support for tearing up the Pinochet-era constitution two years ago in the wake of months of fiery protests against inequality.
The vote is a crossroads for Chile, long seen as a bastion of conservatism and market-orientated economic policy, which underpinned decades of growth and stability that also created stark inequality between rich and poor.
"This is about settling a historical debt in Chile, because despite economic growth and lower poverty, we have outstanding debts to do with inequality and social welfare," said Vlado Mirosevic, spokesperson for the approve campaign.
Mirosevic said the new constitution was key to overturning decades of inequality and put progressive rights and the environment at the heart of the country's social fabric.
Nearly 80% of Chileans voted to draft a new constitution in October 2020. An elected 155-member assembly, consisting of mostly independent and progressive constituents, then began drafting it the following May, completing it earlier this year.
But enthusiasm has waned as Chile's economy has felt the impact of the COVID-19 pandemic, spiraling inflation and the currency hitting historic lows. That's hurt support for the constitution and its backer, progressive President Gabriel Boric.
Polls indicate the reject camp holding a near 10 percentage point lead of around 46% to 37%, according to the most recent surveys. Some 17% remain undecided.
Ximena Rincon, a conservative senator campaigning against the new constitution, said people had lost faith in the assembly responsible for drafting the text. Lots of uncertainty and disinformation has bogged down the constitution also.
"The assembly wasn't representative of society," she said, calling for a smaller, more representative assembly to be elected if the new constitution was rejected on Sunday.
Kenneth Bunker, a political analyst, said Sunday's vote might also act as a referendum on Boric, a young former student protest leader who took office in March and has supported the new constitution.
"There will be people who see the price of gasoline and food, and blame the government over the economy and say that they're the same ones who made the constitution, and so decide 'I vote no'," Bunker said.
Unpredictability remains, however, given the number of undecided voters and a mandatory vote as opposed to previous elections where voting was voluntary.
"That's probably the biggest uncertainty, a lot of people like to extrapolate results from polls," said Rossana Castiglioni, a professor of political science at Diego Portales University.
"But the truth is we know relatively little from this 50%, from this half of the population that abstains from electoral processes."
Boric has said he would launch a new process to draft another constitution if the current one fails on Sunday, while other political factions want to amend the current text. Regardless of the result, experts say Chileans still want the change that they clamored for in 2019-2020.
"People are still waiting for the social agenda that was outlined after the (2019) social uprising, a trend that brought Boric to the presidency," said Axel Callis, a political analyst for pollster TuInfluyes, adding protest anger could be reignited.
"If this doesn't lead to deep changes in terms of social rights, health and pensions, then we're going to be left with an explosive atmosphere."