mexico city mexico rule institutional revolutionary party pri say thursday registration pri contenders july 2018 presidential election begin dec. national convention would pick candidate feb. 18. statement pri say national convention would make around 19,100 delegate choose second week december president enrique pena nieto bar law seek second six-year term rule mexico continuously 1929 pri become byword corruption time vote 2000. pena nieto return party power 2012 slew corruption scandals ongoing gang violence anemic growth sap pri credibility past five years party face uphill struggle hang power veteran leftist andres manuel lopez obrador twice runner-up presidency lead early poll 2018. run-up election take place midst fraught discussions mexico unite state canada future north american free trade agreement nafta underpin much region commerce u.s. president donald trump threaten walk away accord rework favor unite state three nations pledge keep talk march mexican rule party pick presidential candidate february 18