SAO PAULO, Aug 11 (Reuters) - Brazil's President Luiz Inacio Lula da Silva's advantage over Senator Flavio Bolsonaro in a potential runoff ahead of October's general election narrowed, a CNT/MDA poll showed on Tuesday.
Lula would receive 48.0% of the votes in a runoff, compared with 39.1% for Flavio Bolsonaro, son of former President Jair Bolsonaro, according to the poll.
In a June survey, Lula was seen with 49.3% of the votes, and Bolsonaro with 36.8%.
In a first-round scenario, Lula would lead with 42.4% of the votes, virtually unchanged from the June survey, followed by Bolsonaro's 28.7%, Ronaldo Caiado's 4.0%, Romeu Zema's 3.3% and Renan Santos' 2.8%.
If no candidate wins more than 50% of valid votes in the first round, the two frontrunners advance to a runoff.
CNT/MDA interviewed 2,002 people from August 5 to 9 in 140 Brazilian municipalities. The survey's margin of error is 2.2 percentage points in either direction.
(Reporting by Isabel Teles; Writing by Fernando Cardoso; Editing by Chizu Nomiyama and Keith Weir)