By Patrick Markey
BOGOTA (Reuters) - Colombians voted in congressional elections on Sunday in a barometer of May's ballot to replace President Alvaro Uribe, a U.S. ally who finishes two terms dominated by his war on leftist rebels.
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 consider launching 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 this year is seen increasing five-fold to $10 billion (6.6 billion pounds) since Uribe came to power in 2002.
Guerrillas often dominated past elections with bombings, kidnappings and attacks, but Sunday's vote went ahead with little violence, a sign of the success of Uribe's U.S.-financed campaign against Latin America's oldest insurgency.
A strong showing by Uribe's U Party on Sunday will benefit Juan Manuel Santos, a former defence minister who is ahead in opinion polls and who says he is the heir to the campaign against the FARC or Revolutionary Armed Forces of Colombia.
But Santos still does not have the support to avoid a second round in June, and a weak result in Congress could convince other parties in the coalition to abandon a proposal for a unity candidate and present their own presidential bids.
"For Sunday's election and for the May election, we share the same objective, to defend, continue, and improve on the legacy of President Uribe," Santos told a party assembly before the legislative vote.
Initial results are scheduled to be released after 4 p.m. local time 2100 GMT with most of the results expected to be released by 11 p.m. local time or 0400 GMT.
The election race has heated up since February when a court ruled Uribe could not run for a third term. But any candidate to replace him likely will adhere to his popular security and investment policies in Latin America's No. 4 oil producer.
Voters will elect 102 senators and 166 representatives on Sunday to the Congress, which has been marked by scandal over lawmaker ties to outlawed militias. 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.
BATTLE IN THE ALLIANCE
But Cambio Radical has shifted away from the government alliance and its own candidate, veteran lawmaker German Vargas Lleras, is already campaigning for the presidency.
The Conservative Party -- a key alliance member -- also holds its internal election on Sunday 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.
"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."
Authorities detected nine bombing attempts on Sunday and reported heavy fighting with rebels in southern Cauca province where one officer was killed and five soldiers and three police wounded. But voting went ahead peacefully across the country.
The next government will need a majority in Congress to push through health reforms, alongside changes to the pension and tax systems and rigid financial transfers to regional administrations -- important for tackling Colombia's growing fiscal deficit.
Sunday's vote will also test how influential Colombia's armed groups remain in politics.
During Uribe's second term, the Congress was caught up in 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 and kidnapping 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.
(Editing by Cynthia Osterman)
