BRASILIA, Brazil (Reuters) -- Brazil's President Luiz Inacio Lula da Silva could face a battle to win reelection despite apparently weathering a corruption scandal with his popularity intact.

The former metalworker's charismatic appeal has helped him stay popular despite a 10-month-old corruption scandal that flared up again in March and forced Antonio Palocci to resign as finance minister two weeks ago.

However, Lula's main contender, former Sao Paulo governor Geraldo Alckmin, has only been in the race for a month and could gain ground once he becomes better known, analysts say.

The departure of Lula's closest aides to run in elections has also left him more vulnerable to opposition attacks in the campaign. "We are very cautious. This is going to be the tightest election in recent history," said Henrique Fontana, leader of Lula's Workers' Party in the Lower House. "The opposition will radicalize, it will be difficult."

In a weekend poll Lula widened his lead over former Sao Paulo Governor Geraldo Alckmin to 15 percentage points from 12 points in March.

He would beat Alckmin, who is favored by several key business leaders, with 52 to 37 percent in a run-off vote if the election, scheduled for October, were held now, the Datafoha poll showed.

Since June of last year, a scandal in which the ruling Workers' Party used illicit funds to finance campaigns and allegedly bribe legislators has dogged the government.

Opposition politicians said they were not encouraged by the poll result but remained hopeful. -------Rethink of strategy

"We need to rethink our strategy, you cannot run a campaign only on corruption," Gustavo Fruet, a congressman from the opposition Brazilian Social Democracy Party, which is fielding Alckmin, told Reuters.

"But we shouldn't forget Lula was unbeatable ten months ago, today there may be a run-off vote," he said.

Lula has not officially declared his candidacy but is widely expected to do so before a June deadline.

Since the fall of Palocci, the opposition has been attempting to implicate Justice Minister Marcio Thomaz Bastos for supposedly trying to cover-up corruption allegations, such as using confidential banking records to discredit a witness.

The opposition hopes investigating Bastos will keep the scandal alive long enough to tarnish Lula. Bastos said he wants to go to Congress this week to address the accusations.

Lula's supporters say the opposition's current strategy could backfire but admit they remain vulnerable on the corruption issue. "They are going too far with this," said Fontana, adding that his party still needed to convince "voters the irregularities we committed won't happen again."

Corruption is not the only factor that will impact the outcome of the election.

Lula may benefit from an economy that is gaining steam and expected to grow about 3.5 percent this year.

But in addition to Palocci's departure, several key Lula aides stepped down last month to run in October's election, exposing him more directly to opposition attacks.

"The government's ability to fight back may be weakened," said Christopher Garman, Latin America director with the Eurasia Group consultancy in New York.

Alckmin, who has yet to develop nationwide recognition, is strong in the states where voters know him.

"Where Alckmin is known, he is ahead or tied with Lula. This is still an open race," said Garman.