Brazilians saying that President Jair Bolsonaro is doing a "bad or terrible" job rose to 38% from 33% previously, as the government's handling of surging fires in the Amazon rainforest divides the country.
The Datafolha polling institute found that 29% say that Bolsonaro is doing a "great or good" job, down from 33% in July, according to the poll published on Monday.
Brazilians who said the Bolsonaro was doing a normal job fell to 30% from 31% previously, within the margin of error.
The poll diverges from the usually equal three-way split between those approving, disapproving and rating normally the presidency of Jair Bolsonaro, with his negative numbers gaining an upper hand, said Leonardo Barreto, a partner in Brasilia-based consultancy Capital Politico.
"Very likely, it's the people who were giving the president the benefit of the doubt but are losing patience for some reason," Barreto said.
The three factors likely changing Brazilians' minds are the lack of a major economic recovery, Bolsonaro letting up on his aggressive rhetoric on combating corruption and concerns that his government has lost control over the Amazon, he said.
Datafolha found that 51% of Brazilians thought Bolsonaro had done a bad or terrible job responding to the Amazon fires.
Last month, Brazil's space research agency, INPE, revealed that the highest number of forest fires were tearing through the Amazon rainforest since 2010, sparking international outcry demanding that the country do more to protect the world's largest tropical rainforest.
Bolsonaro authorized the military to fight the fires after several days of public protests and criticism from world leaders.
For the first time, farmers who voted en masse for Bolsonaro in the election might be reconsidering their support because of the failure to stop criminals setting fires in the Amazon and fears that it could hurt global demand for Brazilian agricultural products, Barreto said.
"This affects the heart of the Bolsonaro electoral base," he said.
The far-right firebrand engaged in a public war of words with French President Emmanuel Macron, who called for more to be done to combat them as Group of Seven wealthy nations met in France.
Bolsonaro ultimately said that he would only accept an offer of $20 million in aid if Macron withdrew "insults" against him.
Datafolha found that 66% of Brazilians said that Brazil should accept international aide to combat Amazon deforestation.
The Datafolha institute poll surveyed 2,878 people in 175 towns across Brazil on Aug. 29 and 30 and has a 2% margin of error.