Brazil's Rousseff rises in presidential race-poll

* Rousseff widens her lead by 1 point to 24 points

* Would win in first round with 57 percent of valid vote

BRASILIA, Sept 16 (Reuters) - Brazil's ruling party presidential candidate Dilma Rousseff widened her lead over her opposition rival and is set for a landslide victory despite a series of ethics allegations against her, an opinion poll showed on Thursday.

Rousseff garnered 51 percent of voter intention against 27 percent for her main rival Jose Serra of the PSDB party, a Datafolha poll showed.

The same poll last week showed Rousseff ahead with 50 percent against 27 percent.

Former Environment Minister Marina Silva obtained 11 percent support, unchanged from last week, according to the poll published in the online edition of Folha de Sao Paulo.

Surging on the back of the enormously popular outgoing President Luiz Inacio Lula da Silva, Rousseff would obtain the majority of votes she needs to knock out Serra in the first round of voting on Oct. 3.

With null and blank poll responses removed -- as happens with spoiled ballots in the election -- Rousseff had 57 percent, the Datafolha poll showed.

Former Sao Paulo state governor Serra has focused on ethics accusations against Rousseff and her political party in recent weeks, but has been unable to dent the leadership of Lula's former chief of staff.

Serra, 68, accuses Rousseff and her leftist Workers' Party of having illegally accessed tax records of his daughter and opposition members to gather potentially damaging information against them.

In a second scandal, a former Rousseff aide, who is now Lula's chief of staff, is being investigated for her involvement in a kickback scheme for public works contracts run by her son's consulting firm.

Rousseff denies wrongdoing in both cases.

She is widely expected to continue the market-friendly policies of Lula, though some analysts and political allies say she may also expand the role of the state in key areas of the economy.

The Datafolha survey polled 11,784 people between Sept.13 and Sept. 15 and has a margin of error of plus or minus 2 percentage points. (Reporting by Raymond Colitt and Bill Trott)