President Enrique Pena Nieto's PRI came into the June 5 elections holding nine of 12 governors' seats at play, out of 32 federal entities.
But it won in just five states and lost its grip on four that it had controlled for decades -- a result that analysts saw as a punishment by voters fed up with corruption and drug violence.
"Today I present my resignation to allow a new leadership to head the transformations that our party urgently requires," Beltrones said, just 10 months after taking the job.