In Mexico State, Higinio Martínez Miranda has led the faction Grupo de Acción Política since the PRD's founding. While it has much weakened in recent years, due to the rise of the ADN and Nueva Izquierda democratic socialist and social-democratic factions, it remained an important part of the PRD.
Yet now GAP has also declared it is leaving the party. And it does so in what is unfortunately a classic manner: It lost the party's internal elections, and rather than to accept its minority status - and the outcome of the election, of course - it is leaving the party, likely for the PT or Movimiento Ciudadano or, once it becomes an official party, AMLO's MORENA.
At this point in Mexico's political development, a split in the left is very damaging in the short run, but perhaps not so in the long run. Let's recall that the other "left" - and the quotation marks are warranted - parties PT and Movimiento Ciudadano, while bleeding support from the PRD, would likely both have disappeared from the landscape had it not been for AMLO allowing them to use him for electoral benefit.