Now that moderate Jesús Ortega has been declared the winner of the March contests for the PRD presidency, the extremist wing of the party loyal that inclines to El Peje is making noise that they are going to break from the rest. Evidently, the pejistas are negotiating an alliance with the smaller leftist parties that are now part of the Broad Progressive Front in advance of the summer elections.
It seems like this is the end of a single leftist party as a major force in Mexican politics. Another will certainly reemerge, but one wonders what it'll look like: a further left version of today's PRI that includes Ortega and his clan; a more permanent version of the coalition mentioned above, which could shove Ortega's wing into the PRI; or an Ortega-centered party that includes neither the PRI or the more extreme wing of the left (which would struggle mightily for votes).
As you can see, I think where Ortega and his kind wind up will be the key indicator. I really don't have a clue which of the above scenarios will win out, or how long it will take. I am somewhat suspicious of the PRI's ability to gain a lot of support on the left, because most of the left's intellectual leadership grew in opposition to the PRI. Essentially, the memory of the authoritarian, anti-democratic PRI is too fresh for it to be a credible leftist party for a good ten years or so.