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BRASILIA (Reuters) - Former Sao Paulo Mayor Fernando Haddad would surge to second place in Brazil’s presidential race if he were to be backed as the Workers Party candidate by jailed former President Luiz Inacio Lula da Silva, a new poll showed on Friday.

Far-right candidate Jair Bolsonaro was the front-runner with 21 percent of voter support, according to the poll, conducted June 4-6 by research institute Ipespe. Haddad tied with environmentalist Marina Silva for second with 11 percent.

The poll, which surveyed 1,000 voters by telephone, was commissioned by Brazilian brokerage XP Investimentos and published by the Valor Economico newspaper. The telephone polling has raised doubts in a country where not everyone has a phone.

Haddad’s support surged from 3 percent to 11 percent with those surveyed when his name was linked to the support of Lula, who is still Brazil’s most influential politician despite a corruption conviction that will bar him from running.

Ipespe director Marcela Montenegro said the aim of the poll was to anticipate Haddad’s growth in support when Lula drops his plan to run and backs the former mayor as expected.

Centrist reformer Geraldo Alckmin, the former governor of the state of Sao Paulo, polled just 8 percent irrespective of mention of Lula, while center-left candidate Ciro Gomes rose from 9 percent to 11 percent if the survey did not mention Lula backing Haddad.

The poll showed Bolsonaro losing to Marina Silva by 35 percent to her 36 percent in a runoff that will be held if no candidate wins a majority in first round voting on Oct. 7.

Bolsonaro would defeat Gomes by 35 percent to 33 percent in a run-off and Alckmin by 34 percent to 29 percent, while Gomes would knock out Alckmin by 32 percent to 29 percent and Alckmin -would beat Haddad by 30 percent to 20 percent, the poll said.

Haddad lost his 2016 bid for re-election as Sao Paulo mayor in an unprecedented first-round defeat by Joao Doria of Alckmin’s centrist party.