The PRI's Slide

Two takes on the polls that show the PRI dropping, starting with Leo Zuckermann, who thinks that the decline of the PRI in the polls is due to the PAN's aggressive negativity:

I would dare to speculate that the negative campaign from the PAN against the PRI is working. It would be that a segment of the population started to doubt its vote for the PRI. The panistas, above all in the voice of their national leader, Germán Martínez, have cornered the priistas. They have created a wave of news where, over the internet, with repetition in national news organizations, they ask the PRI what side it's on in the war against organized crime. On the president's or who else's? The "else" meaning the drug traffickers. If the PRI answers the president, then it merely scores points for the PAN. And of course for obvious reasons they can't say that they're on the side of the criminals.

Ricardo Raphael wonders if the party's decline is because the PRI never poll as well with more people voting/responding:

A chain from which the PRI has never managed to escape would seem to be crystalizing once more: while this party tends to win elections in which few people participate, they are defeated when the number of voters grows.

The poll from Berúmen and Associates seems to confirm this: when 6 percent of those polled didn't want to offer information reasoning the vote was secret, the PRI obtained 40 percent of the voting preferences. In contrast, once more people involved themselves in the exercise, the PRI lost a fourth of the voting intention that it enjoyed before.

Of the two explanations, I think Zuckermann's is more likely the main cause.