Brazil’s congress has approved a constitutional amendment to shorten the working week to a maximum of 40 hours, a move that could affect 30 million workers and that will hand a political victory to the leftist president Luiz Inácio Lula da Silva ahead of the presidential runoff in under two weeks.

In an attempt to overturn the first-round lead secured by his opponent, far-right senator Flávio Bolsonaro, Lula has strongly championed the amendment, which will give some workers an extra day off each week.

Bolsonaro, by contrast, spent months opposing the change but reversed course on Tuesday, saying “the majority of Brazilian workers” supported it. On Wednesday, he joined 75 senators who approved the reform, which passed the lower house in May.

Only one senator voted against it: Hamilton Mourão, former vice-president to Jair Bolsonaro, Flávio’s father.

The constitutional amendment bans six-day workweeks, currently worked by 16 million people, while cutting the maximum workweek from 44 to 40 hours for more than 30 million workers.

The change is expected to affect mainly workers in retail, restaurants and hotels, most of them low-paid – one of the groups which Lula lost the most support from since the 2022 election, when he defeated Jair Bolsonaro.

Lula celebrated the amendment’s passing, writing on social media “ending the 6x1 schedule is about more than changing working hours. It means more time to live, rest, be with family, watch your children grow up, look after yourself and enjoy life.”

Victor Escobar, a researcher at the Laboratory of Parties, Elections and Comparative Politics, said the change “could indeed benefit Lula’s re-election campaign”.

The proposal began as a TikTok movement called Life Beyond Work, founded by a pharmacy assistant who entered politics after its success and later was embraced by Lula.

His party spent months pushing the senate to put the proposal to a vote and many believed that after the far right’s sweeping victory last week secured a broad congressional majority in 2027, the Senate would no longer vote on it.

But Senate president Davi Alcolumbre, whose relationship with Lula has at times been rocky, put it to a vote.

Flávio Bolsonaro’s party had considered blocking it, but polls showed the change had more than 70% of the general population behind it, and more than half of the senator’s voters.

In May, Bolsonaro said the proposal could cause “mass unemployment”, an argument employers’ associations opposed to the change have repeatedly made.

The senator voted remotely while campaigning in the country’s south.

Even after changing his position, Bolsonaro said on Tuesday that he “supports something even better,” referring to an alternative proposal he had long advocated in which pay would be based on the hours actually worked. The Folha newspaper reported that he told business leaders that, if elected, he would review parts of the new amendment, a point several allied senators also made during Wednesday’s vote.

Despite its approval, none of the changes will take effect before the runoff on 25 October. After the Senate president promulgates it, a 60-day period will follow before six-day workweeks are abolished and the maximum working week is reduced from 44 to 42 hours, then to 40 hours after a further 14 months.

However, Escobar believes its approval could already benefit Lula: “It will speak to how voters feel economically when they cast their ballot; they can feel more relieved with a change in their working schedule”.

Marilane Oliveira Teixeira, a professor and researcher at a study centre at the University of Campinas who has spent months examining the possible impacts of the change, said there was no shortage of evidence showing how harmful having only one day off is to workers.

“People who work a 6x1 schedule are practically prevented from having even a minimum level of social life,” she said, adding that the negative effects could include illness, workplace risks and exhaustion, “especially for women, who still carry the burden of unpaid domestic work”.

Lilian Cristo dos Santos, 50, has only one day off from her job as a saleswoman at a home appliances shop in Salvador, Bahia. “There’s no time to go to the doctor, to the beach, to the salon, to look after myself; I wanted to have time to study and I can’t. On my only day off, I have to clean the house and look after my family,” she said.

Santos said she voted for Lula, but has colleagues who chose Bolsonaro. “They say he represents change – well, yes, but certainly for the worse.”

Escobar is among many political scientists who believe that, despite Bolsonaro’s lead, Lula has gained momentum not only from the change to the working schedule, but also from demonstrations by his supporters that have drawn thousands across the country and an effort to build a stronger presence on social media.

Lula also continued announcing benefits just days before the vote, including debt relief for microentrepreneurs, and published a letter to evangelical Christians – who have historically favoured Bolsonaro – stressing his commitment “to ensuring that everyone has two days off a week, because life is not just about work”.

Escobar said: “Although most people have already decided their vote, there are some who are more responsive to political moves, and Lula has been trying very strongly to reach them. … The race still looks somewhat tangled, tied, with some degree of advantage for Bolsonaro, but it is still a very open election.”