The decision by Cuevas, who is head of the Senate’s foreign relations committee, represents a win for López Obrador as he seeks to oust the ruling Institutional Revolutionary Party (PRI) in his third attempt to clinch the presidency.
“I’ve decided to join the movement that Andrés Manuel López Obrador has created,” Cuevas told reporters as she announced her departure from the party she had been with since she was 15.
In a statement, the PAN said Cuevas chose to leave after it was unable to guarantee her a federal representative’s position in the future.
The PAN has entered an electoral alliance with the centre-left Party of the Democratic Revolution (PRD), with former PAN leader Ricardo Anaya at its head.
The coalition has put a squeeze on the number of safe seats the PRD and PAN can each distribute among leading politicians.
López Obrador has vowed to combat inequality and corruption. But some international investors are concerned by suggestions that he might reverse parts of the government’s 2013-14 energy legislation, which ended a decades-long monopoly by state oil firm Pemex.
Anaya’s coalition is second in opinion polls.
PRI contender José Antonio Meade is in third place, weighed down by the PRI’s poor record on corruption.