BOGOTA (Reuters) – Colombian presidential frontrunner Juan Manuel Santos faces a struggle to secure an outright victory in May’s election after legislative election results over the weekend put President Alvaro Uribe’s alliance under pressure.

Uribe’s U Party, led by Santos, and the allied Conservative Party scored a majority in Colombia’s parliament on Sunday. But the solid Conservative showing is likely to dampen proposals the party join Santos under a unity candidate.

The vote was a barometer for the May 30 ballot to succeed Uribe, one of Washington’s top allies who is popular at home for his security drive against leftist guerrillas and applauded by Wall Street for his pro-investment policies.

Colombia’s peso currency closed yesterday up 0.34 percent at 1,893.5 versus the dollar as investors sold dollars purchased as a precaution before the election. It was another sign investors are expecting stability during the handover from Uribe in Latin America’s No. 4 oil producer.

Uribe’s U Party secured 27 seats in the 102-member Senate with the Conservative Party winning 23, bringing the pro-Uribe coalition a strong majority in the upper house, according to official results. In the 166-member lower house, the U Party was set to have won around 43 seats while the Conservative Party secured 47, according to tallies from regional authorities with more than 50 percent of the vote counted.

Santos, a former defence minister closely associated with Uribe’s security successes, is ahead in opinion polls and is trying to rally the coalition around a unity candidate.

But he does not have the more than 50 percent support needed to avoid a runoff in June, polls and analysts say.

“This is a mandate from the Colombian people,” Santos told Caracol Radio. “The campaign for the presidency starts now.”

Colombia’s election race heated up after a court ruled in February that Uribe could not run for re-election after two terms highlighted by his hard line against the FARC or Revolutionary Armed Forces of Colombia.

Uribe’s coalition consists of the U Party, the Conservatives, Cambio Radical and a host of smaller parties in both houses. But Cambio Radical is also already campaigning with its own presidential candidate, German Vargas Lleras.

“There is not going to be a president in the first round,” said Jaime Castro, a former Bogota mayor who is now a political analyst. “All the parties and their candidates will go to the first round to test their strength.”

Any candidate replacing Uribe is unlikely to steer too far from his security and free-market policies that have increased foreign investment from $2 billion when he came to office in 2002 to $10 billion expected this year.