Enrique Peña Nieto, who polls show is the favorite to win Mexico's July 2012 presidential election, vowed over the weekend to work for unity.

"We are going to win by doing good politics, mature and constructive politics, politics that contributes, politics that transforms realities," Peña Nieto told supporters after officially registering Sunday as the only candidate of the Institutional Revolutionary Party, or PRI.

Peña Nieto said he would work for party unity, reaching out to PRI members and leaders.

"The PRI's strength is in its leadership, its members and supporters," the 45-year-old Peña Nieto said.

The former governor of Mexico state said he would promote ideas and compromise to win the backing of Mexicans, while avoiding the provocations of the opposition.

Mexico's causes "will be the PRI's causes," Peña Nieto said.

"There is fear, anxiety, discouragement, but at the same time there is a growing force, optimistic and sure that better times" are coming to Mexico, the PRI candidate said.

"It is clear that the PRI is more alive than ever," Peña Nieto said, adding that party members have "an opportunity and a historic challenge."

The PRI, which governed Mexico from 1929 to 2000, ended up as the No. 3 party in the 2006 election.

"Mexico wants to leave division and anger, the weakening of our institutions and the deterioration of the values of our society, behind," Peña Nieto said.

Peña Nieto announced his bid for the PRI's presidential nomination on Sept. 20, a few days after ending his term as governor of Mexico state, which surrounds the Federal District and forms part of the Mexico City metropolitan area.

The former governor was given a clear path to the nomination last week when Sen. Manlio Fabio Beltrones, the only other candidate vying to head the ticket in the 2012 elections, withdrew from the race.

Mexico will hold its presidential election on July 1, 2012, electing a successor to President Felipe Calderon of the National Action Party, or PAN.

The leading candidates are Peña Nieto, who will be the standard-bearer of the Compromiso por Mexico coalition formed by the PRI, the Mexican Green Party, or PVEM, and the New Alliance Party, or PANAL; and former Mexico City Mayor Andres Manuel Lopez Obrador, who heads the ticket of the alliance formed by the leftist Party of the Democratic Revolution, or PRD, the Workers Party, or PT, and the Movimiento Ciudadano.

The PAN, which is going it alone in the election, has not decided on a candidate yet.

Josefina Vazquez Mota, Santiago Creel and Ernesto Cordero are vying for the PAN's nomination.

Some 80 million Mexicans will be eligible to vote for a new president, 628 legislators and thousands of other officials in next year's general elections.