By Helen Murphy
BOGOTA (Reuters) - Colombia's President Juan Manuel Santos heads into presidential elections for a second straight term with his lead all but certain, even while seats for his ruling coalition were trimmed in a congressional vote that cemented his fiercest rival as the main opposition, results showed on Monday.
With most of Sunday's congressional vote counted, Santos' backers had won enough seats in the Senate and lower house to support ongoing peace talks with Marxist FARC rebels and provide a snapshot of national support for the center-right president.
One-time ally turned rival, former President Alvaro Uribe became the biggest opposition force in the legislature, winning 19 seats in the Senate for his Democratic Center party - including one for himself - against 21 for Santos' U Party. In the lower house, the U won 37 seats against Uribe's 12.
Despite losing clout from his coalition in congress, the result bolstered confidence among Santos' backers that he could win a second term in presidential elections on May 25.
"We can win in the first round," said Sergio Diaz Granados, head of Santos' U Party. "Peace is what will help generate employment, improve security and advance the social reforms the country needs."
Although right-wing Uribe garnered a large chunk of support in congress, it may be tough to transfer that backing to his presidential candidate, former finance minister Oscar Ivan Zuluaga.
Zuluaga has struggled to gain momentum against Santos, picking up 10.8 percent of the vote in a recent Gallup poll against 34.7 percent for the president. A candidate needs to win more than 50 percent of the ballot to avoid a runoff in June.
Still, Uribe's presence thins Santos' support for steering reforms through the legislature and may complicate the implementation of a peace deal with FARC, or Revolutionary Armed Forces of Colombia, if talks succeed.
With almost 99 percent of votes counted, 62-year-old Santos' center-right U Party emerged the single biggest party in both congressional houses.
But counting all the seats that make up his coalition - the Conservative, Liberal, Radical Change and U parties - Santos lost some of clout.
Santos wants a second term to allow him time to complete negotiations with the FARC that could end a war that has killed more than 200,000 and transform Colombia's political makeup if the rebels' gain the political participation they seek.
Uribe, 61, is a bitter critic of the government who believes the FARC should instead be beaten militarily. His party will likely seek to obstruct legislation if a peace deal is reached that would enable FARC rebels to enter the political system without serving considerable jail time.
Santos called on Uribe to put aside the "hatred" and work in congress for good of the nation.
"There was a very important signal sent to the nation and the whole world, that grand majority of Colombians want peace," Santos said late on Sunday after the results were revealed.
The secretive peace talks reached a partial accord late last year on the FARC's participation in politics, a highly controversial item on the five-point agenda. Any deal with the rebels would be put to the nation in a referendum, and then to congress to devise laws for its implementation.
Despite slow but encouraging progress at the negotiations in Cuba's capital Havana that began in late 2012, the decision to engage in peace talks with the guerrillas remains divisive and will be pivotal in voters' choice of president in May.
Uribe became the de facto opposition and Santos' fiercest critic shortly after backing him for office in 2010.
The two fell out when Santos mended ties with Venezuela's then-President Hugo Chavez, who had engaged in a diplomatic tussle with Uribe for years. The acrimony worsened when Santos announced peace talks with the FARC, seen as a terrorist group by the United Sates and the European Union.
"Compatriots, today the Democtratic Center Party was born. It won't waver before terrorism," Uribe said on his Twitter account.
Colombia, a recipient of hundreds of million of dollars in annual U.S. anti-narcotics aid, has fought the FARC, right-wing paramilitaries and a smaller rebel group, the ELN, since 1964. More than 200,000 people have died and millions have been displaced.
Santos is expected to reveal soon that the ELN will also start peace talks with his government, which is likely to give a further boost to his chances of securing another term.
Santos will also need backing in congress to pass reforms that would help bolster Colombia's $350 billion economy, create new jobs and cut the poverty rate, which affects about half the nation's population of 47 million.
(Additionary reporting by Peter Murphy and Luis Jaime Acosta Editing by W Simon)