Colombians are set to head to the polling stations for electing a new congress as major political parties choose candidates for the May election to succeed President Alvaro Uribe.
Uribe's ruling Conservative party is likely to keep its majority in both houses, which currently stands at 72 out of 102 seats in the Senate, and 103 out of 166 in the House of Representatives.
Uribe's conservative administration is Washington's top ally in South America both in the fight against regional drug trafficking and in trying to counter the growing influence of Hugo Chavez, the Venezuelan president.
After two terms in office - ending in August - Uribe's nurturing of the economy to its best performance in 30 years and his crackdown on the leftist rebel Revolutionary Armed Forces of Colombia (Farc) have earned him a near 70 per cent popularity rating.
But the Colombian congress has been plagued by scandals, with scores of legislators investigated for alleged links to paramilitary groups, including 12 who were convicted.
Sunday's elections, it is hoped, will wipe the slate clean, but the Electoral Observation Mission has already reported $3.5m of illegal contributions for one senate race alone.
The Organisation of American States (OAS) has also warned that drug cartels could influence the elections with their mountains of cash.
A top OAS official told El Tiempo daily: "We've heard many voices and all seem to concur that still there are criminal groups - not just the paramilitaries - directly linked to drug trafficking that are trying to have a perverse influence in politics.”
Enrique Correa, chief of the OAS election observer mission, said in an interview published on Saturday: “In Colombia there is still a risk drug trafficking will try to influence politics, as it does in the entire world."
One political party has already been struck from the voting list on suspicion its leaders had links to paramilitaries, though opposition groups claim the party simply renamed itself and kept the same people, or their relatives, in charge.
Some 150,000 military and police have fanned out across Colombia to safeguard some 77,000 polling stations.
About 29.8 million Colombians are registered to vote to choose from 2,539 candidates to fill congress' combined 268 seats.
Five members will also be elected to the regional Andean Parliament.
Polling stations will open from 8:00am (1200 GMT) to 4:00pm (2000 GMT).
Sunday's vote will also kick off internal consultations in the major Conservative and Green parties to pick a candidate for the May 30 presidential election - with a runoff, if needed, on June 20.
Uribe's absence from the presidential race - his bid for a third term in office was blocked last month when the Constitutional Court ruled a referendum on the issue unconstitutional - has left his supporters without a clear leader.
Among half a dozen Conservative members vying to be candidate, two are most likely to succeed: Uribe's former ambassador to Britain Noemi Sanin and ex-agriculture minister Andres Felipe Arias.
Other political party hopefuls for the May election include former defence minister Juan Manuel Santos, 58, an Uribe supporter. Surveys have him leading the pack, followed by leftist Gustavo Petro, pro-Uribe Radical Change Party's German Vargas Lleras, and independent Sergio Fajardo.