The candidate supported by Brazilian President Luiz Inacio Lula da Silva has been elected as the new leader of the governing Workers' Party.
President Lula's party is deeply divided along ideological lines
Ricardo Berzoini, a cabinet minister, beat his left-wing rival, Raul Pont, in a close second round of voting.
The result has yet to be announced officially, but Mr Pont has already acknowledged defeat.
But many members did not vote in what has been seen a sign of disenchantment with a corruption scandal.
Correspondents say many of them are also opposed to the government's market-friendly economic policies.
Some had threatened to leave the party if the ruling faction remained in control.
Former party leader Jose Genoino was forced to step down amid allegations of illegal campaign financing and corruption.
The scandal also led to the resignation of three other senior officials.