By Helen Murphy and Brian Ellsworth
BOGOTA, May 26 (Reuters) - President Juan Manuel Santos and opposition challenger Oscar Ivan Zuluaga scrambled on Monday to win the support of candidates eliminated from Colombia's presidential election and gain the upper hand ahead of a runoff vote next month.
Zuluaga, a right-wing former finance minister, won most votes in the first round of voting on Sunday after campaigning to roll back Santos' peace talks with Marxist guerrillas. He now faces Santos in a runoff on June 15.
Both men immediately began seeking alliances and wooing supporters of the three candidates knocked out of the race on Sunday. All three were in consultations with their parties to determine who they will back.
Zuluaga met on Monday with third-placed candidate Marta Lucia Ramirez, who is also skeptical of the peace talks and could throw her support behind him. She won 15.5 percent of the vote on Sunday, and Zuluaga described their meeting as "very positive".
Clara Lopez and Enrique Penalosa, who came in third and fourth with a combined 23.5 percent of the vote, support negotiations with the FARC and so may be more inclined to side with the president.
Seeking their support, Santos said all three of the losing candidates had made "important proposals" that could be made "reality together" over the next four years.
Zuluaga won 29.3 percent support in the first round, 3.6 percentage points more than Santos.
He has run an aggressive campaign against Santos' peace talks, but in an apparent effort to win over moderates he offered on Monday to put a limit on prison sentences for rebel leaders of the Revolutionary Armed Forces of Colombia (FARC).
"FARC leaders who have committed atrocious crimes, crimes against humanity who should spend 50 years in jail, I'm willing to reduce their sentences to six years," the 55-year-old economist told Colombian media.
The FARC has refused to accept prison terms or the unilateral ceasefire that Zuluaga is demanding.
Negotiations in Cuba to end Colombia's 50-year civil war appear to be in limbo until the election is decided.
Santos, a 62-year-old center-rightist and U.S. ally, has staked his presidency on the peace talks, saying they offer the best hope of ending a war that has killed 200,000 people and for decades hamstrung Latin America's fourth largest economy.
For months, he had a comfortable lead in polls but Zuluaga surged in the last three weeks, backed by popular former President Alvaro Uribe, who turned against Santos over his decision to negotiate with the FARC.
While many voters care more about the economy than the war, there is not much difference between the pro-business policies of Santos and Zuluaga. So the runoff will, like the first-round vote, essentially be a referendum on Santos' strategy.
Zuluaga, 55, accuses Santos of giving too much away at the secret negotiations, and wants a return to the hardline military strategy Uribe pursued during his eight years in power.
He has vowed to give the rebels eight days to lay down their weapons and suspend negotiations if they refuse. That appeals to those Colombians outraged that FARC leaders could be spared prison time and allowed to run for Congress under a peace deal.
A regional breakdown of Sunday's votes showed strong support for Santos in areas of continuing conflict while Zuluaga won in areas that benefited from Uribe's U.S.-backed military offensives. (Additional reporting Julia Symmes Cobb, Nelson Bocanegra and Peter Murphy; Editing by Kieran Murray and Andrew Cawthorne)