Nearly 80 million Mexicans are eligible to cast ballots Sunday in an election expected to result in the return to power of the once-dominant Institutional Revolutionary Party, or PRI, after a dozen years in opposition.

The final opinion polls, published Wednesday, showed the PRI's Enrique Peña Nieto with a comfortable lead over his three rivals in the race to succeed Felipe Calderon, constitutionally barred from seeking a second term.

Peña Nieto's advantage over leftist candidate Andres Manuel Lopez Obrador ranged from 10 to 18.6 percentage points.

Josefina Vazquez Mota, candidate of the governing conservative National Action Party, or PAN, stood in third place, with support of around 24 percent, while Gabriel Quadri, of the tiny New Alliance Party, registered in single digits.

The official 90-day presidential campaign drew to a close on Wednesday.

Frontrunner Peña Nieto ended his campaign in Toluca, a city in central Mexico that is a bastion of his party.

"We are ahead in all the polls, but we cannot allow ourselves to let our guard down. This is the time to redouble our efforts," the 45-year-old candidate said.

The PRI, which governed Mexico from 1929 to 2000, is trying to regain the presidency after losing the last two elections to the PAN.

"We are four days away from winning the presidency of the republic," Peña Nieto said, adding that Mexico needed "responsible change."

Lopez Obrador also cited the need for change, telling tens of thousands of supporters at a rally in Mexico City's Zocalo plaza that he would win.

"The people want real change and they will not be able to prevent it with dirty wars or by buying loyalties, consciences and votes," he said.

The former Mexico City mayor told his supporters that he campaigned hard so he "can again win the presidency of the republic."

Lopez Obrador lost the 2006 presidential election to the PAN's Calderon by 0.56 percent and has never recognized the results.

Josefina Vazquez Mota, of the PAN, said that if she won on Sunday, she would ask Calderon, who made the war on Mexico's drug cartels the centerpiece of his administration, to serve as her attorney general.

"I want an attorney (general) for the nation who is not an accomplice of crime," she said during a rally at Estadio Omnilife in Zapopan, a city in the western state of Jalisco.

Quadri - the longest of longshots - ended his campaign in Zacatecas, saying that he wanted Mexicans living in the United States to return home, create businesses and spur development.

The day after the official close of electioneering, all four presidential candidates pledged to accept the results of Sunday's balloting and to firmly reject acts of violence.

The hopefuls signed the pact in a ceremony at the Federal Electoral Institute, whose chairman, Leonardo Valdes, said the accord marks a commitment to respect "the will of the citizens as expressed at the polls."

Much of the political discussion has centered on whether to continue Calderon's militarized approach to the stuggle with cartels, a strategy that has cost more than 50,000 lives since he took office in December 2006.

Even so, a U.S. think-tank says it is Mexico's poor economic performance since 2000 that will seal the defeat of the governing PAN in the presidential election.

The Center for Economic and Policy Research analyzed Mexico's economy over the past decade, including the 2008-2009 recession, and concluded that "the country's economic failure is a main reason for the likely loss of the ruling PAN party."

"Mexico's economy has done terribly since 2000, and for the last three decades, by any comparison - compared to its past growth (e.g. 1960-1980), or even compared with the rest of Latin America, which has grown twice as fast in per capita GDP since 2000," Mark Weisbrot, lead author of the paper and CEPR co-director, said.

Besides electing a successor to Calderon, Mexicans will choose among candidates vying to fill 628 congressional seats and thousands of state and local offices. EFE