* Polls show Fajardo is a contender if Uribe does not run

* Known for solid administration as Medellin mayor

(Adds details of another Uribe ally candidacy)

By Patrick Markey

MEDELLIN, Colombia, June 25 (Reuters) - His casual, blue-jeans style contrasts sharply with Colombian President Alvaro Uribe's buttoned-down image, but Sergio Fajardo believes he can replace the incumbent in next year's election.

While Uribe considers a 2010 re-election bid, Fajardo, a former Medellin mayor praised for turning around a city that was once a byword for violence, is steadily going about the business of promoting his candidacy.

Polls show Uribe is a Goliath, outweighing any rival just 10 months from the May election, even though he would need a change in the constitution to allow him to seek a third term.

Scores of would-be candidates wait in the wings -- including allies of the president -- but when pollsters take Uribe out of the election equation, Fajardo proves to be a strong contender.

A U.S.-educated math professor who sees parallels in U.S. President Barack Obama's grass-roots campaign to rejuvenate politics, Fajardo says Colombia needs to break a cycle of corruption and political polarization.

"Colombia needs strong moral leadership, and that is the big challenge. Here there is huge corruption, where we have lived in a society enclosed by violence," Fajardo said. "What is needed is a new start. That is our proposal."

Reluctant to discuss concrete policies for now, Fajardo, 53, acknowledges he sounds idealistic talking about a fresh start in a country marked by four decades of guerrilla war.

Transparency in government would be one key focus and he acknowledges the success of Uribe's popular U.S.-backed security policies, but sees a need for a strong emphasis on social and economic development.

In his Medellin office, a table is scattered with newspapers and notepads with diagrams of proposals. Fajardo, the son of an architect, reveals his academic past by scribbling ideas on a white board to make a point.

His major hurdle is Uribe, who many Colombians say saved a state that once teetered on the edge of failure. But he sees room for a new approach as Uribe's supporters -- "Uribistas" -- and opponents squabble over his future.

"I have always said, neither Uribista nor anti-Uribista. It can't be about just that," Fajardo said.

URIBE CHALLENGE

Uribe's security drive has battered the FARC rebels and he negotiated the disbanding of 30,000 outlawed paramilitaries who once fought guerrillas and carried out scores of massacres, sometimes in league with the military.

But Uribe's second term has been marred by scandals over lawmakers' links to paramilitary death squads, illegal wiretapping by state security agents and investigations into troops who murdered civilians to count them as combat deaths.

Uribe's supporters are now pushing for a constitutional amendment to allow his re-election. But critics worry that another Uribe term would undermine the country's democracy.

One former Uribe ally, German Vargas Lleras, announced his candidacy late on Thursday, promising to maintain the president's security policies. Uribe's defense minister Juan Manuel Santos has already resigned to run himself if the president decides against re-election.

A recent Gallup poll showed Santos tied with Fajardo as the most favored candidates barring Uribe.

Already some see similarities with Fajardo's and Uribe's first candidacy -- both men are natives of Antioquia region, known for its work ethic. Both stepped onto the national stage as outsiders but with a solid record as local administrators.

Fajardo says comparisons end there. He says he has always believed in a negotiated end to the conflict, but he says lessons from failed talks with rebels must be considered.

His key pitch is his 2003-2007 administration as mayor of Medellin, where he helped build cable car lines, schools and libraries for poor barrios while offering training programs to draw young people out of violence.

Fajardo dismisses critics who claim Medellin's security benefited more from Uribe's success and the criminal control exerted by a former paramilitary warlord. But he says his experience can be applied to the national level.

"With or without Uribe, we are going," Fajardo said. "I'm a mathematician and problems, you must understand them, look at them, study them, so when we say something, we do it." (Editing by John O'Callaghan and Chris Wilson)

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