The party known as the PRI governed Mexico for most of the last century, and for all of the past five years.
But sagging polls standings due to violence, corruption, high inflation and weak economic growth have the PRI looking to Jose Antonio Meade, a former treasury secretary who is not a party member.
Meade got the support of most of the party's leaders at an event on Sunday at which he registered to seek the party's presidential nomination for the July 2018 election.
READ: Mexico finance minister launches presidential run
He is all but guaranteed the nomination, after his only real competition - former Yucatan state Governor Ivonne Ortega - said on Saturday that she wouldn't compete and would instead back Meade.
Far from distancing himself from the PRI, Meade has embraced unfashionable old party rituals like rallies with old-guard union bosses and farm leaders, and events with bused-in crowds.
Meade stuck close to the line of current President Enrique Pena Nieto, telling a crowd at the party headquarters in Mexico City that continuity is the best bet.
"Not everything has to be demolished, not everything has to be changed, not everything has to be invented," Meade said.
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