In May 1987, he won a seat in the 1987 election as one of two opposition members in the country’s 24-member Senate (the other being Joseph Estrada), finishing 24th. He was unable to be proclaimed until August, when the electoral protest filed by Augusto Sanchez was dismissed.
He formally assumed office on August 15, 1987. In the same month, an attempted coup against Aquino escalated and led to the destruction of the Armed Forces General Headquarters (AFPGHQ) in Camp Aguinaldo in Quezon City. He was detained in Camp Aguinaldo over suspicion of planning the coup with Lt. Col. Gregorio Honasan, but was released days later for lack of evidence.