UPDATE 2-Rousseff steady in Brazil poll before last debate

* Debate on TV Globo may be last opposition chance

* Supreme Court decision on voting rule may favor Rousseff (Updates with Supreme Court ruling in last 3 paragraphs)

By Raymond Colitt

BRASILIA, Sept 30 (Reuters) - Brazilian ruling party candidate Dilma Rousseff has just enough votes to win Sunday's presidential election, a poll showed on Thursday, hours before a TV debate in which her rivals have a final chance to force her into a runoff vote.

The poll by the Datafolha firm showed the career civil servant and former chief of staff to President Luiz Inacio Lula da Silva with 52 percent of valid votes, up from 51 percent in a similar poll published on Tuesday.

That was well down from her peak of 57 percent two weeks ago, around the time a corruption scandal forced the resignation of a former aide, but above the 50 percent threshold she needs to avoid a runoff on Oct. 31.

Her closest challenger, former Sao Paulo state Governor Jose Serra of the centrist PSDB party, has struggled to connect with voters and failed to take advantage of Rousseff's loss of momentum in recent weeks. He had 31 percent of valid votes in the latest Datafolha poll, slipping from 32 percent. Valid votes exclude null and spoiled ballots captured by pollsters.

The latest poll of 13,195 people was conducted on Tuesday and Wednesday and had a margin of error of 2 points. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Full coverage of election: [ID:nBRAZIL]

Election Top News page: link.reuters.com/dux43p

Graphic on polls: link.reuters.com/vux47n

Candidates' main proposals: [ID:nN29180225]

Political risks in Brazil: [ID:nRISKBR]

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Rousseff, a former leftist militant who recovered from cancer last year, faces a last hurdle on Thursday night in a closely watched debate on TV Globo, the country's largest and most influential television network. If elected, she would become Brazil's first female president.

Rousseff, 62, has rarely strayed from a narrow script throughout the campaign as she rides the wave of Lula's popularity. She is expected to play it safe in the debate, focusing on Brazil's economic boom and government social programs that have helped pull more than 20 million people out of poverty under Lula's presidency.

"The opposition would have to present something entirely new and bombastic to throw her off course," said Alexandre Barros, a Brasilia-based political consultant.

"Warming up old corruption allegations won't do it."

FINAL TV DEBATE

Emboldened by recent gains in polls, third-place former Environment Minister Marina Silva is likely to join Serra in renewing attacks against Rousseff over corruption allegations involving her aide and the ruling Workers' Party.

Although Silva dipped a point to 15 percent in the Datafolha poll, she has risen 5 points in the past month.

TV Globo reaches the most distant corners of this continent-sized country with the biggest audience of any network, although the debate's timing after the last soap opera program at 10:30 p.m. could put off many viewers.

The final debate has been seen as potentially important since a poor performance by Lula in 1989 turned the tide in favor of rival Fernando Collor de Mello in Brazil's first direct presidential election since the end of military rule.

It was the first of three elections that former metalworker Lula lost before winning in 2002. Brazil has since cast off its unstable reputation and become one of the world's fastest-growing emerging markets.

A decision on voting procedures by Brazil's Supreme Court on Thursday could further benefit Rousseff. The top tribunal voted to relax a rule that required voters to present both a registration card and photo identity card at the polling booth. Now voters will only need the photo ID.

That may benefit poorer and less-educated voters, who often lack proper documentation and make up a large proportion of Rousseff's supporters. (Editing by Stuart Grudgings and Peter Cooney)