BRASILIA, Brazil (Reuters) -- Less than two weeks before Brazil's presidential elections, renewed allegations of wrongdoing involving the ruling Workers' Party could put President Luiz Inacio Lula da Silva on the defensive and dent his solid lead in the polls.

The latest accusations follow last year's campaign financing scandal led by his party. Lula had recovered from the scandal to take a lead of more than 20 points in opinion polls over former Sao Paulo state Gov. Geraldo Alckmin of the opposition Social Democracy Party, or PSDB.

The federal police now are investigating claims that the Workers' Party paid for a dossier that would implicate Alckmin and Jose Serra, PSDB candidate for Sao Paulo governor. The dossier reportedly included documents, photos and a DVD related to kickbacks on the purchases of ambulances.

Police arrested two men in Sao Paulo on Friday with about 1.7 million reais ($790,000) in cash, which investigators suspect was earmarked to pay for the dossier. One of the two is a member of the Workers' Party.

An adviser to Lula, who had been named as a possible Workers' Party interlocutor in the case, stepped down on Monday. He denied any wrongdoing but told Globo Television that Lula was very concerned. A presidential spokesman confirmed the adviser's resignation.

The Workers' Party denied it paid for a dossier.

But the PSDB and other opposition parties officially called on the Supreme Electoral Tribunal on Monday to open an inquiry. They argue the incident represented undue government interference in the election campaign.

"I have no doubt that this will change the course of the election," said Sen. Heraclito Fortes, one of Alckmin's campaign managers of the allied Liberal Front Party.

Authorities said investigations into the incident would not be concluded before Oct. 1 but depending on their outcome, could still affect election results.

"If it was proven the (dossier) was set up, there will be serious consequences," said Marco Aurelio de Mello, the top election authority.

Opposition parties on Monday said the case was an opportunity to attack Lula and hurt his chances of winning the simple majority needed to avoid a run-off vote.

"If we use it well, it could lead Lula to lose the four percentage points needed for a run-off vote," Sen. Alvaro Dias of the PSDB told Reuters.

At the weekend the PSDB already lambasted the Workers' Party in its televised campaign ads. "Where did this money come from? Who is behind this dirty game?" an announcer said. Lula said at the weekend that the attempted purchase of a dossier against the PSDB was "abominable" and that widespread use of dossiers against candidates was "an absurd aspect of Brazilian politics."

Part of the allegations against Serra was published in a news magazine last week. They said the PSDB and Serra supported a scheme in which legislators received kickbacks on sales of ambulances they financed through budget amendments. The magazine provided no proof.