One is a far-right populist who wants to clean up Brazil, the other a leftist political heir to popular – but imprisoned – ex-president Luiz Inácio Lula da Silva.
Come October 28, Brazil's 147 million voters will have to decide which one will be their next president.
Sometimes called a tropical Trump for his pro-gun stance, rhetorical swagger and a canny use of social media akin to the US president's approach, far-right Jair Bolsonaro has built an image as a political outsider ready to rough up the establishment.
That's quite a feat, for unlike Donald Trump, Bolsonaro is a longterm politician.
This seven-term congressman has few legislative initiatives to his record but has made enemies with his intolerant comments putting down women, gays and blacks, while talking fondly of the 1964-1985 military dictatorship he served as an Army captain.
"The dictatorship's mistake," he said two years ago, "was to torture and not kill."
Bolsonaro has promised, however, that if elected he would govern "with authority, but not authoritarianism."
Born 63 years ago to a Catholic family with Italian roots, Bolsonaro served in the military before starting his political career in 1988 as a Rio municipal councilor.
Two years later, he was elected to the federal Chamber of Deputies, where he has been since.
Over one relationship and two marriages he has fathered five children: four sons (three of them politicians) and – in what he called a moment of "weakness" – a daughter.
The leftist Workers' Party (PT) tapped Haddad late in the game to replace its preferred candidate, Lula, who is serving a 12-year prison term for graft.
Initially, Haddad sold himself as "Lula's man" to tap into the former president's broad popularity. That helped boost him in the polls and out of relative obscurity as a former mayor of São Paulo and, before that, education minister in Lula's government.
But after Brazil's first-round election this year – and amid incessant attacks by Bolsonaro highlighting past corruption in the Workers' Party – Haddad has tried to carve out his own profile while reducing Lula's image in his campaign.
More recently, he began emphasizing what he says is the threat Bolsonaro poses to Brazilian democracy.
An academic with degrees in law, economics and philosophy, Haddad, 53, had hoped to make up ground by publicly debating Bolsonaro.
But his far-right rival has spurned that idea, preferring to duel over Twitter and Facebook.
Haddad, a keen guitar player, is married to a dentist; they have two children. He told the daily El País two years ago he saw himself as a "political being," steeped in public life since his university days.
"I'm not an anxious person," he said. "I wait for things to happen to make decisions."
Bolsonaro, perhaps unsurprisingly, has chosen a recently retired general, Hamilton Mourão, 65, as his vice-presidential pick.
Perhaps also unsurprisingly, Mourão himself has a history of making controversial comments while extolling the righteousness of the military.
He has made racist remarks about Brazil's indigenous population – from which he is partly descended – and blacks.
And last month, he caused an uproar when he said children brought up by single mothers tended to join drug gangs.
Haddad's running-mate couldn't be more different. Manuela d'Avila, 37, studied journalism and is a state lawmaker in the Communist Party of Brazil.
A self-styled "feminist and revolutionary," she sharply defended herself from critics when she was photographed two years ago breastfeeding her baby daughter in her state legislature.
D'Avila, too, was endorsed by Lula, who called her a representative of the new generation on the political left.
She was a federal deputy from 2007 to 2015, winning election to a second term with the most votes of anyone in the chamber.