UPDATE 3-Brazil's Rousseff widens lead, may avert runoff

* Rousseff stretches poll lead to 20 points

* Threatens strongholds of challenger Serra

* First-round win could boost her mandate (Adds opposition accusations, paragraphs 14-15)

By Guillermo Parra Bernal

SAO PAULO, Aug 26 (Reuters) - Brazil ruling party candidate Dilma Rousseff surged ahead of her main rival in his home state bastion, an opinion poll showed on Thursday, as she looked headed for a mandate-boosting outright win in the Oct. 3 election.

The Datafolha survey showed Rousseff overtaking Jose Serra in Sao Paulo and in other southern states that had been thought to be competitive for the veteran opposition candidate.

Her lead over the former Sao Paulo state governor has jumped since the start of slick campaign slots on television last week that have linked her strongly to popular President Luiz Inacio Lula da Silva.

The ads have given many voters their first in-depth look at Lula's former chief of staff, who wants to be Brazil's first woman president and pledges a mix of a strong state role and market-friendly policies to build on recent economic success.

Rousseff, of Lula's ruling Workers' Party, had 49 points in the latest Datafolha poll, and Serra had 29. That compares with her 47-30 lead in the previous Datafolha survey released six days ago.

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For a factbox on recent polls, click on: [ID:nN24259710]

For a graphic on all polls: link.reuters.com/vux47n

Rousseff comments on budget cuts: [ID:nN24265576]

A story on the TV campaigns: [ID:nN26128430]

Factbox on political risks in Brazil: [ID:nRISKBR]

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Support for Rousseff in Sao Paulo jumped to 41 percent from 34 percent, while Serra shrank 5 points to 36 percent.

Her lead in Minas Gerais, considered a vital "swing" state, rocketed to 19 points from 6 points in the last poll. Serra also slipped behind in southern Rio Grande do Sul, considered an opposition stronghold.

"The poll shows a very serious threat to Serra," said Amaury de Souza, an analyst with MCM Associates consultancy in Rio de Janeiro.

Rousseff's surge in southern, richer states showed her appeal was spreading nationally and to different economic classes, beyond the poor northeast that has for years been Lula's stronghold, he said.

OPPOSITION STRUGGLE

Thursday's poll was the third in five days to show Rousseff heading for the majority of votes she needs to be elected president without a run-off election. It showed her getting 55 percent of valid votes once spoiled and blank ballots are discarded, as they are on election day.

Rousseff, 62, had trailed Serra in national polls, including Datafolha, as recently as June.

The former leftist militant has been shown in the TV ads, which are crucial opinion-formers in Brazil elections, as a determined, compassionate leader who promises to "continue changing" Brazil.

The opposition has appeared to struggle to find a consistent line of attack against Rousseff and has even tried to jump on the Lula bandwagon by including images of Serra with the president in its television ads.

The tone turned nastier on Thursday as Serra's opposition parties said they would sue Rousseff for electoral crime after national newspapers reported that bank details of three opposition officials had been illegally accessed.

The opposition says it was an attempt by Rousseff's camp to build damaging evidence against the opposition. The Workers' Party said it in turn would sue Serra for "irresponsible accusations."

Not even Lula, a charismatic former union leader who now basks in approval ratings around 80 percent, managed to win in a first-round vote in the 2002 and 2006 elections.

A first round victory might give Rousseff a stronger mandate to push through her legislative agenda which includes revamping oil, natural gas and mining laws to boost government revenue and the state presence in those sectors, analysts have said.

Green Party candidate Marina Silva's support in the poll fell to 9 percent from 10 percent.

Datafolha polled 10,948 people nationwide between Aug. 23 and Aug. 24. The poll has a margin of error of plus or minus 2 percentage points. (Writing and additional reporting by Stuart Grudgings; editing by Mohammad Zargham)