Jair Bolsonaro being stabbed during a campaign rally.
He suffered a deep and life-threatening wound to his intestines and lost 40% of his blood.
He will be confined to either hospital or his home for several more weeks.
The stabbing boosted his popularity from 26% to 30%.
Bolsonaro is a divisive figure, not dissimilar to Trump, with Brazilian evangelicals supporting his anti-abortion stance.
Former President Lula da Silva, who until recently was well ahead with 39% in the polls, is widely expected to pull out of the race later on Tuesday. Lula, who is in jail serving a 12-year sentence for corruption and money-laundering, has been barred by Brazil's highest electoral court from running. His legal team is still appealing against the decision but sources close to his Workers' Party told Reuters news agency that he would announce on Tuesday that his running mate, Fernando Haddad, would replace him.