* Vote may reveal Santos support in presidential election
* Uribe in Senate may seek to block FARC peace laws
* Peace talks remain key battleground
By Helen Murphy and Luis Jaime Acosta
BOGOTA, March 9 (Reuters) - Colombians vote on Sunday for a new congress that will tackle legislation for the next president and play a historic role in creating laws to end five decades of conflict if peace is reached with Marxist FARC rebels.
The ballot is likely to consolidate President Juan Manuel Santos as the frontrunner for a second straight term in a presidential vote on May 25, allowing him to continue talks with the Revolutionary Armed Forces of Colombia (FARC) that could end the war and transform Colombia's political makeup.
"This election is different, it's distinct. It's the most important congressional election in 50 years," said Senate candidate Juan Manuel Galan. "Congress not only receives the mandate of the people, but the mandate of peace."
Up for grabs are 168 seats in the lower house and 102 in the Senate. One of the Senate seats is being sought by Santos' right-wing predecessor, Alvaro Uribe, a fierce critic of the government who believes the FARC should be beaten on the battlefield.
The still-popular Uribe is expected to win the Senate seat easily, raising the temperature in congress as he may seek to block legislation that could enable FARC rebels to enter the political system without serving considerable jail time.
Secret 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.
Uribe, 61, and his backers accuse Santos of offering rebel leaders soft sentences, or none at all, to cement his place in history as the leader who brought peace to Colombia. Santos denies the FARC will get off scot-free.
"The role of congress is going to be very important post-conflict and in the construction of peace after any accord is signed," said Alejo Vargas, a political science professor at the National University in Bogota.
"At least one sector of the right opposes (a peace deal) because it believes surrender or a military end is more desirable. ... The composition of the new congress will determine how much of an obstruction the right could be."
URIBE FEARS FARC IMPUNITY
Some 32 million Colombians are eligible to vote, though congressional elections have a particularly high abstention rate. The government said it will deploy 260,000 members of the armed forces to maintain security during the election.
Formerly a Santos ally, Uribe's Democratic Center party may well draw votes away from the government's center-right alliance of parties, which has supported Santos over the last four years.
While Santos, 62, has solid backing from about 80 percent of lawmakers, including the Conservative, Liberal, Green, Radical Change and U parties, Uribe could dilute that.
The ex-president 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.
"I'm afraid of what will happen if an impunity pact is signed with terrorist leaders," Uribe said at the close of his campaign. "When crime is a champion, there's no condition in the heart to forgive the criminal. The lack of justice may lead to peace accords in Havana but more violence in Colombia."
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.
Polls show Santos is likely to reach a second round of voting on June 15 with Oscar Ivan Zuluaga, the candidate for Uribe's party. The top two contenders go to a runoff if neither garners more than 50 percent of ballots cast in the first round.
Santos will 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 remains at about half the nation's population of 47 million.
"It's crucial for Santos and his government to know what legislative support they will have going into a second term," said Vargas, the university professor. "That will have repercussions on how the new government is formed."
(Additional reporting by Andres Rojas and Camilo Cohecha in Bogota; Editing by Daniel Wallis and Jonathan Oatis)