By Luis Jaime Acosta

BOGOTA (Reuters) - Colombia's former President Alvaro Uribe, revered by many for crippling the power of the country's guerrillas, said on Monday he plans to return to politics by seeking election to the Senate in March.

Head of state from 2002 to 2010, Uribe left office with high ratings and has since been a fierce critic of his successor and former defence minister, President Juan Manuel Santos.

The 61-year-old trained lawyer is constitutionally prohibited from running for a third presidential term.

However, he could make a forceful return to politics by leveraging his popularity to help his nascent Democratic Centre movement enter Congress due to a list-based system of voting that would enable him to pass surplus votes to other members of his party.

"We want to help confront the deterioration in security, growing uncertainty for investment, stagnation in a bureaucratic social policy, the waste of official resources ...," he said, adding there was a risk of "delivering the nation to terrorism."

Uribe, a conservative and pugnacious politician who was a close ally of the United States while he was in power, was speaking to reporters at a press briefing at his residence near the northwestern city of Medellin.

Political progress for Uribe and his party in the Senate has the potential both to weaken Santos' grip in parliament and strengthen the opposition's hand before the May 2014 presidential election.

In swipes often delivered via Twitter, Uribe accuses Santos of frittering away security gains and giving into terrorism since entering last November into peace talks with the Revolutionary Armed Forces of Colombia (FARC) rebels.

"Authority, institutional legitimacy and the confidence of the worst-off sectors are disappearing, while social discipline is being lost," Uribe said.

Santos must decide by November whether he will run for a second term in the presidential poll scheduled for next May. His popularity has taken a battering, slipping to 21 percent according to Gallup, after a nationwide farming sector strike that caused widespread disruption.

(Writing by Peter Murphy; Editing by Andrew Cawthorne and Cynthia Osterman)