By Hugh Bronstein

BOGOTA, April 12 (Reuters) - Colombia's left-wing opposition is trying to capitalize on a scandal tying allies of conservative President Alvaro Uribe to right-wing paramilitaries, a gambit filled with risks for both sides.

Sen. Gustavo Petro, a one-time member of the disbanded M19 rebel group, says he will reveal the names of Uribe supporters who helped the "paras" fight Marxist guerrillas, often using brutality to terrorize civilians.

Eight of Uribe's congressional allies are already in jail on charges of collusion with paramilitaries guilty of some of the worst massacres of Colombia's four-decade-old rebel war.

Opening a campaign ahead of October provincial elections, Petro is using the issue to establish his Polo Democratico as the "anti-Mafia party" in a country where paramilitary drug smugglers have infiltrated all levels of society.

"I have a list of 2,000 people who were involved, but I am only going to reveal the most important names," Petro said.

He will focus a congressional debate scheduled for Tuesday on the growth of paramilitary groups in Antioquia during the 1990s, when Uribe was governor of the province.

"This is part of the Polo Democratico's strategy to position itself as the alternative to Uribe not only in this year's elections but also in 2010 when the next president will be elected," political commentator Ricardo Avila said.

After years of being dragged down by voters concerns that it was linked with still active guerrilla armies, Polo Democratico came in second in last year's presidential race.

Uribe won reelection by a landslide in the vote, however, and enjoys approval ratings above 70 percent, thanks largely to his U.S.-backed crackdown on the rebels.

But more investigations into links with paramilitaries are expected and the scandal risks damaging his standing abroad.

Democrats in control of the U.S. Congress say Colombia must improve its human rights record as a condition of continuing military aid and approval of a free trade deal.

KILL THE MESSENGER?

Petro, who has received death threats, does not have an easy job ahead of him.

"Colombian voters tend to punish both the accuser and the accused, because they don't like being reminded that their country is corrupt," said local pollster Napoleon Franco.

"Petro is taking a risk. It will be hard for him to convince common voters that the Polo is clean while the other parties are not," Franco said.

The paramilitaries were organized in the 1980s to help protect property from left-wing rebels. By the late 1990s, the fight had become little more than a turf war over cocaine smuggling routes in which thousands are killed every year.

Colombia is the world's biggest producer of the drug.

Most paramilitary leaders have turned in their guns over the last three years as part of a peace deal in which they and 31,000 of their fighters surrendered in exchange for benefits including reduced jail terms.

But human rights groups say paramilitary chiefs have kept control of their criminal networks from behind bars. The government admits that thousands of former militia fighters have regrouped into new crime gangs.

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