Serra holds lead in Brazil presidential race-poll

* Lula-backed Rousseff jumps to 25 pct from 17 pct

* Opposition candidate Serra leads with 36 pct

* Poll likely to bolster Serra camp

BRASILIA, Feb 18 (Reuters) - Sao Paulo state Governor Jose Serra, the principal opposition candidate in Brazil's Oct. 3 presidential election, holds a large but narrowing lead over the ruling party contender, an opinion poll showed on Thursday.

Serra of the opposition PSDB party had 36 percent of voter intention, down two percentage points from November, according to a survey by polling firm Ibope.

President Luiz Inacio Lula da Silva's chosen candidate, his chief of staff Dilma Rousseff, jumped to 25 percent from 17 percent in November.

Lula's ruling Workers' Party is expected to endorse Rousseff as its candidate at a convention this weekend.

The poll was commissioned by the Sao Paulo Trade Association and published in its Diario do Comercio newspaper.

A survey by polling firm Sensus published on Feb. 1 had shown Serra's lead shrinking to only 33-28 percent against Rousseff.

"[Serra] is still the favorite. Without campaigning his standing remained unchanged," Marcia Cavallari, executive director of planning at Ibope, told the Diario do Comercio. Serra's two percentage point fall was within the margin of error of the poll, she said.

The poll is likely to bolster spirits in the Serra camp, where advisors had become concerned Roussseff was close to drawing even.

Rousseff has been benefiting from a growing economy and Lula's enormous popularity, analysts said.

Latin America's largest economy bounced back from a brief recession in the second quarter of last year and is forecast to grow by more than 5 percent this year.

Neither of the two leading candidates in the October election are likely to stray far from Lula's economic policy, which includes a free-floating currency, inflation targets and a primary budget surplus to service public debt.

The Ibope poll also showed former Ceara state Governor Ciro Gomes, with 11 percent support, down two percentage points from November.

Former Environment Minister Marina Silva, who is expected to run for the Green Party and vie with Rousseff for votes from leftists and women, had 8 percent of voter intention, up from 6 percent.

The survey polled 2,002 people between Feb. 6 and 9.

(Reporting by Raymond Colitt; Editing by Bill Trott)