RIO DE JANEIRO — By the time Brazilians were done voting Sunday, mighty power brokers had been tossed out of office, long-dominant political parties had been humbled, and a far-right populist suddenly looked like he just might be the most powerful man in the country.
It was, in short, the most sweeping political shift Brazil had ever seen in a single election since democracy was restored in 1985.
“What we are watching today is the collapse of our current system,” said Maria Hermínia Tavares de Almeida, a political scientist at the University of São Paulo.
The near-winner in the first round of voting in the presidential race was Jair Bolsonaro, a former army captain. He offered few detailed policies. But his draconian approach toward fighting violence — he would make it easier to for the police to kill suspected criminals and imprison more people for longer — appealed to many in a nation traumatized by rising crime, a dispiriting economy and a political class widely regarded as venal and unresponsive.
Mr. Bolsonaro’s show of “discipline and strength” has attracted middle-class voters, said Thiago Aragão, a political consultant based in Brasília. “They treat Bolsonaro as a last resort, as someone who could contain the growth of violence in Brazil,” he said.
When they embraced him, sending him to the second round with 46 percent of the vote, Brazilians sent an unmistakable message: They want a drastic course correction. And if a measure of authoritarianism is required, so be it.
This is a shocking reversal for Brazil. When the country emerged from two decades of military rule in 1985, the population celebrated the end of a regime that had killed or disappeared 434 people and tortured thousands.
In 2002 Brazilians elected as president Luiz Inácio Lula da Silva, a former metal worker who rose through union ranks as he fought to bring down the dictatorship. He was followed in office by Dilma Rousseff, his handpicked successor and a former guerrilla who was tortured under the dictatorship.
The Workers’ Party governed until 2016, when it was swept out of office after the impeachment of Ms. Rousseff amid a painful recession and a widespread investigation into graft that tainted much of the country’s political leadership.
That series of traumas has made some older Brazilians look back to the military years with nostalgia — a sentiment openly embraced by Mr. Bolsonaro, who says he intends to name military officers to his cabinet. This is a huge shift in a country where the military has largely stayed out of politics for 30 years.
Brazilians are not alone in their demand for radical change. Support for democracy in Latin America is at its lowest level in more than a decade, according to the pollster Latinobarómetro. In Brazil in 2017, only 13 percent of the population said they were very satisfied democracy — far below the average of 30 percent for the region. Elsewhere in the world, frustration with the status quo has fueled the rise of populists with open disdain for democratic institutions like a free press and an independent judiciary.
As Mr. Bolsonaro and his opponent in the election, Fernando Haddad of the leftist Workers’ Party, geared up on Monday for the second round of votes on Oct. 28, Brazil’s political establishment was struggling to make sense of the political earthquake that had just rattled the nation.
Candidates affiliated with Mr. Bolsonaro performed far better than polls had suggested, an indication he could have robust support in Congress and among elected officials across the country if he becomes president.
The Social Liberal Party, which Mr. Bolsonaro joined earlier this year to launch his presidential bid, became a behemoth overnight, expanding its footprint in the lower House of Congress from eight seats to 52. That makes it the second-largest party in that chamber, trailing only the Workers’ Party, which lost five seats.
There was also shock over the success of a regional candidate who was virtually unknown until Mr. Bolsonaro threw his weight behind him in the last days of the race.
Polls had suggested that the candidate, Wilson Witzel, a retired federal judge and military veteran running for governor of Rio de Janeiro, had no shot at victory over his better-known opponent. But with Mr. Bolsonaro’s backing, Mr. Witzel won 41 percent of the vote, more than twice his opponent’s tally.
As they took stock of the new political landscape, political scientists and academics said Monday that the Bolsonaro phenomenon may have forever changed how campaigns are run in Brazil.
Unlike rivals who had far more time on national television — which is awarded based on party size — and who bought polished ads, Mr. Bolsonaro ran a scrappy, inexpensive campaign powered mainly by social media. Supporters created hundreds of group chats on the messaging app WhatsApp, which the vast majority of Brazilians use, sharing campaign information, jokes, memes and conspiracy theories.
Victor Piaia, a sociologist who studies political communication at the State University in Rio de Janeiro, said it was unclear how much the campaign had coordinated with chat groups. But he said it was clear it had played a role in steering the narratives and benefited from the platform’s message-amplifying effect.
“This type of communication is less top-down,” said Mr. Piaia. “Everyone is the curator of their own content, and this makes the information distributed there more appealing.”
Mr. Piaia said he was especially surprised at how successful the Bolsonaro campaign was in directing down-ballot votes. While other political parties were handing out pamphlets on the streets, it spent weeks distributing a message with the names of candidates it supported through WhatsApp groups.
Among the biggest losers on Sunday were elected officials who have been embroiled in the far-ranging corruption investigation known as Lava Jato, or Car Wash. According to the newspaper Folha de São Paulo, 47 politicians who have been charged with corruption, or are being investigated, were not re-elected.
Among them was the president of the Senate, who is being investigated for corruption and money laundering, and a former governor and minister who was caught on a wiretap telling a colleague they needed to find a way to “stop the bleeding,” an apparent reference to the toll the corruption investigation was taking on the political elite.
Mr. Bolsonaro’s promise to take a zero-tolerance approach to corruption has extraordinary appeal for some voters who have come to see their leaders as kleptocrats. But on Monday, some of the candidate’s supporters said fears that he could be despotic were overblown. And some suggested that law-abiding Brazilians need not worry.
Adriana Giotto, 77, a retired lawyer, said she had no traumatic memories of the military dictatorship.
“Those who are good citizens were not affected by the dictatorship in a negative way,” she said. “The dictatorship made mistakes and killed many people, but it was those who choose to live outside the rules of social harmony who set the stage for that to happen.”
Carolina Cremonez da Silva, 31, said that kind of mind-set terrified her.
“Bolsonaro legitimizes violence, authoritarianism” said Ms. da Silva, a psychologist. “He ends up getting many supporters because people are afraid. They are people who want that iron fist, that authoritarian regime, without understanding what that represents for society.”