BRASILIA (Reuters) - Far-right candidate Jair Bolsonaro leads the field for Brazil’s presidential race in October ahead of environmentalist Marina Silva, but one third of voters do not know who to back in the most uncertain election in decades, a new poll showed on Sunday.

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With jailed former leftist president Luiz Inacio Lula da Silva barred from running by a corruption conviction, many of his supporters said they would spoil their ballot or not vote for anyone, the survey by Datafolha pollster said.

A crucial factor in the Oct. 7 election will be which candidate will be backed by Lula, who is still Brazil’s most influential politician even behind bars. Datafolha found that 30 percent of voters will cast their ballot for his candidate.

Right-winger Bolsonaro, a former army captain who favors easing gun controls to fight crime, appears to have gained some traction from supporting a recent truckers strike for lower diesel prices that paralyzed Brazil.

He now has 19 percent of voter support, two percentage points more than in the last poll in April, while Marina Silva’s backing is unchanged at 15 percent.

Center-left candidate Ciro Gomes moved up one point to 10 percent and centrist Geraldo Alckmin, the former Sao Paulo governor favored by investors for his pro-business policies, remains stagnant at 7 percent.

On Friday, Lula’s Workers Party held a convention in his absence and stuck to its plan to register his candidacy, but electoral authorities are expected to keep him off the ballot.

Potential Workers Party stand-ins, former Sao Paulo Mayor Fernando Haddad and former Bahia state Governor Jaques Wagner, only have 1 percent of voter support, Datafolha said, and 61 percent of those polled said they would reject Lula’s candidate.

Marina Silva and Ciro Gomes would benefit from Lula’s absence on the ballot, though Gomes, the former governor of northeastern Ceará state, is seen as the most likely standard bearer of Brazil’s left with the ability to unite other parties behind him.

President Michel Temer’s has become Brazil’s most unpopular president on record and 92 percent of voters surveyed said they would reject the candidate he backs, Datafolha said. Temer’s pick is former Finance Minister Henrique Meirelles, who has not been able to breach 1 percent of voter support in the polls.

Datafolha surveyed 2,824 voters across Brazil between June 6-7. The poll has a margin of error of 2 percentage points.