No party has held a majority in Mexico's Congress since 1997. The PRI has a very good chance of winning one, along with its Green Party allies in the senate, and a shot at a majority in the house of deputies. This would be bolster the prospect of reform in the country. For example, one of the most necessary reforms is in the energy sector, particularly trying to get more private investment into the state-run industry. But that requires a constitutional amendment.
President Calderón tried to amend the existing law, but it many elements of it were blocked by the PRI. Peña Nieto is now supporting many of those same reforms. He will have an easier time if he doesn't have to negotiate with other parties. The PRD is against a significant overhaul of the way the oil sector works, while the PAN could find itself with an interesting role to play: will it negotiate with the PRI to get the reforms it originally wanted passed? Or will it block those reforms to make victory elusive for the PRI?