The leader of the efforts for the "no" vote has been Álvaro Uribe, a former president and current senator whose father was killed by FARC. Much of his argument is emotional; if the referendum went through, FARC guerrillas would not be fully punished for all of their crimes, which Uribe refers to as amnesty. FARC would also become a political group.
What is not reported as much is that the FARC peace process is mirrored on one that Uribe pioneered, when he convinced Autodefensas Unidas de Colombia (AUC), a violent right-wing paramilitary organization dwarfed by FARC, to disarm. Perhaps the difference is personal. For years, Uribe has been hounded by allegations that he is intimately connected with AUC. Reports from the CIA, the Defense Intelligence Agency, and Pablo Escobar's widow have all put him near or at the top of AUC's command structure, and his ties to the organization were investigated by Colombian authorities as recently as 2013.
Uribe was also behind a hugely controversial extradition deal in which paramilitary leaders, including some with AUC, were put to trial in the U.S. and received relatively lenient sentences.