Uribe's allies are seeking to build on their majorities in both houses of Congress and shore up his coalition, which threatens to break apart as member parties squabble over whether to present their own presidential candidates.

The conservative leader remains popular after taking the fight to FARC guerrillas and drug traffickers. Colombia's war has ebbed and foreign investment has increased to $10 billion per year from $2 billion since Uribe came to power in 2002.

A strong showing by Uribe's U Party will benefit Juan Manuel Santos, a former defense minister who is ahead in presidential opinion polls and positions himself as the successor to the campaign against the FARC or Revolutionary Armed Forces of Colombia.

But a weak result could convince other parties in the coalition to abandon a proposal for a unity candidate and launch their own presidential bids.

"Today's elections will give a clearer panorama of the strength of the groups in the running for May 30," the newspaper El Tiempo said in an editorial. "One race finishes today but another begins." The election race has heated up since February when a court ruled Uribe could not run for re-election. Any candidate to replace him likely will adhere closely to his popular security and pro-investment policies.

Voters will elect 102 senators and 166 representatives on Sunday. Candidates range from veteran party chieftains and former hostages of FARC rebels to soap opera celebrities and national soccer players.

Uribe's alliance, made up of his U Party, the Conservative Party, Cambio Radical Party and a group of smaller parties, holds a majority of 68 seats in the Senate and a 107-seat majority in the lower house.
 

But Cambio Radical has shifted away from the government alliance and its own candidate, veteran lawmaker German Vargas Lleras, is already campaigning.

The Conservative Party - a key alliance member - also holds its internal election to decide a presidential candidate, which could result in a splinter for the coalition if the winner decides to face Santos rather than support him.

The next government will need a majority in Congress to push through health service reforms, alongside changes to the pension and tax systems and rigid financial transfers to regional administrations - all important for tackling the deficit.

Sunday's vote will also test how influential Colombia's armed groups remain in politics.

During Uribe's second term, there was a scandal tying dozens of lawmakers to paramilitaries who smuggled drugs and massacred peasants in the name of counter-insurgency before disarming under his government.

Violence from the war has dropped sharply but a report by the national ombudsman said illegal armed groups are still a risk to voting in a third of Colombia's more than 1,000 municipalities.