Former President Alvaro Uribe is back in elected office, and his winning of a Senate seat could potentially complicate efforts by Colombia's government to make peace with leftist rebels.
Uribe's party was the number 2 vote-getter in the Senate in congressional elections behind the party of President Juan Manuel Santos, whose governing coalition lost some ground.
Uribe's conservative group which is opposed to peace talks with FARC rebels, will now be one of the most powerful in Congress.
The former leader has vehemently opposed a negotiated end to a 50-year-old internal conflict that would let the Western Hemisphere's biggest remaining rebel army enter political life.
Santos initiated the talks and hopes to soon conclude them successfully.
Uribe's new Centro Democratico party looks to have won 20 of the Senate's 102 seats on Sunday against 47 for Santos' weakened coalition. Santos' Partido de la U got 21 seats, according to official results with 97 percent of the vote counted.
The coalition's fate will depend on the Conservatives, who won 18 seats.
But it is not clear if they will stay with Santos or join Uribe, who as president in 2002-10 seriously weakened the leftist Revolutionary Armed Forces of Colombia military with close US intelligence and logistical assistance. Santos was defence minister in 2006-09.
If the Conservatives choose Uribe's camp, he could seek to erect legislative obstacles to any peace deal. Santos is the current front-runner in the May presidential election.
The leftist Polo Democratico party lost ground, sliding from eight Senate seats to five. The Green Alliance held at five seats. Lower house results were still being compiled.