* Rousseff coalition has at least 49 of 81 seats in Senate
* Gov't coalition headed for gains in lower house as well
* Result makes approval of gov't oil reform likely in 2010
* Rousseff's challenge to ensure unity within coalition
By Raymond Colitt
BRASILIA, Oct 4 (Reuters) - Brazil's ruling coalition won a large majority of seats in Congress, near-final results showed on Monday, which would boost front-running candidate Dilma Rousseff if she wins a runoff for president later this month.
Such an advantage in Congress would give Rousseff enormous leverage to approve her legislative agenda, which includes plans to simplify the tax code, increase government control over oil and mining and take steps to improve the operating environment for businesses.
Rousseff, once a Marxist guerrilla and outgoing President Luiz Inacio Lula da Silva's former chief of staff, finished a strong first in Sunday's election but will face runner-up Jose Serra of the opposition PSDB party in a runoff on Oct. 31.
In the Senate, where two-thirds of the seats were up for election, Rousseff's 10-party center-left coalition took at least 49 of the 81 seats, or just over the 60 percent majority needed for constitutional amendments.
With the likely support of two undecided parties, Rousseff could have as many as 60 seats in the Senate.
In the Chamber of Deputies, the lower house of Congress, preliminary vote counts showed Rousseff's ruling Workers' Party, or PT, and its allies chalking up strong gains as well. A final tally is expected later on Monday.
"If Rousseff wins, as we still believe, she'd have a comfortable advantage in Congress," said Rafael Cortez, political analyst with Tendencias consultancy in Sao Paulo.
Like Rousseff, many legislative candidates in her coalition benefited from Lula's enormous popularity and a booming economy.
Rousseff's PT and the allied, centrist PMDB saw the largest gains in the Senate. The big losers were opposition parties like the right-wing DEM party and the centrist PSDB of Serra and former president Fernando Henrique Cardoso, who served from 1995 to 2002. Several of the PSDB's best-known senators failed to get re-elected.
If Serra were to be elected on Oct. 31 he would have difficulty forming a clear majority in Congress, despite the likely migration of centrist legislators to his camp.
The government's strong showing also means a bill that would strengthen government control over the oil industry is more likely to get final approval in Congress before the end of the year.
Brazil's parties, however, are notoriously undisciplined and keeping her broad coalition in line would be a major challenge for Rousseff, who had never run for political office before this year.
Rousseff has vowed to try to overhaul Brazil's tax regime, one of the world's most onerous and complex. She said she could seek changes including capital investment and payroll tax breaks, as well as a harmonization of state value-added taxes.
Rousseff, 62, also could relaunch bills stuck in Congress to speed up antitrust rulings, streamline government procurement and cap public sector pay rises, albeit modestly.
She is expected to endorse framework legislation being drafted and likely to be presented to Congress this year that would restrict exploration concessions in the mining industry.
Its approval would clear the way for discussions of a possible royalty hike, authorities say, though the Rousseff camp has denied it has such intentions. (Editing by Bill Trott)