Millions of voters in Mexico are casting ballots for a new president and congress amid allegations of fraud and predictions that an old party is about to stage a comeback after 12 years in the political wilderness.

Polls have opend in Mexico with more than 80 million people eligible to cast ballots.

The Institutional Revolutionary Party (PRI), led by telegenic former Mexico State Gov. Enrique Pena Nieto, has held a strong lead throughout the campaign, and also seems poised to retake at least a plurality in the two houses of Congress.

The party has been bolstered by voter fatigue with a sluggish economy and the sharp escalation of a drug war that has killed roughly 50,000 Mexicans over the past six years. The desire for change suddenly works to benefit the party, known as the PRI, that ran Mexico from 1929 to 2000.

Hoping for a shocking upset are leftist Andres Manuel Lopez Obrador, whose narrow loss in Mexico's last election led to charges of voter fraud and weeks of massive protests, and the candidate of the ruling National Action Party, Josefina Vazquez Mota, the first woman ever nominated for the presidency by a major party in Mexico.

Mexican presidential front-runner Enrique Pena Nieto represents the party known as the PRI, that ran Mexico from 1929 to 2000. (Edgard Garrido/Reuters)

It would be a once-unthinkable comeback for the PRI, which many believed was doomed after its 2000 loss and which was still reeling in the last presidential election, when it finished a weak third.

Pena Nieto has cast himself as a pragmatic economic moderate in the tradition of the last three PRI presidents of the 20th century. He has called for greater private investment in Mexico's state-controlled oil industry, and has said he will try to reduce violence by attacking crimes that hurt ordinary citizens while deemphasizing the pursuit of drug kingpins.

All of the parties are accusing rivals of emulating the traditional PRI tactic of offering voters money, food or benefits in return for votes. Lopez Obrador's Democratic Revolution Party says Pena Nieto's campaign has handed supporters prepaid money cards worth nearly $5.2 million US.

"Where do they get so many resources to conduct the PRI campaign, so many billboards?" asked voter Marilu Carrasco, a 57-year-old actress who was lined up to cast her vote for Lopez Obrador in Mexico City's Copilco neighborhood. The PRI's return to the presidency "could be the worst thing that could happen to us," Carrasco said.

PRI activists have published photographs of truckloads of handouts they say were given out by Democratic Revolution backers.

But electoral officials have repeatedly insisted that outright fraud is almost impossible under the country's elaborate, costly electoral machinery.

The government also promises efforts to avoid outbursts of violence linked to the country's endemic drug gang violence.

Military and civilian officials announced that the army would step up election day patrols in the border city of Nuevo Laredo, where a bomb in a pickup truck exploded outside city hall on Friday.

The 45-year-old Pena Nieto, who is married to a soap opera star, also has been dogged by allegations that he overspent his $330 million US campaign funding limit and has received favorable coverage from Mexico's television giant, Televisa.

University students launched a series of anti-Pena Nieto marches in the final weeks of the campaign, arguing that his party hasn't changed since its days in power.

Promising a more open PRI

Pena Nieto says his party has abandoned the heavy-handed ways of the past and will govern in an open and pluralistic manner, and many say the PRI would not be able to reimpose its once near-total control even if it wanted to because of changes in society, the judiciary and Congress.

"The context has changed dramatically," said Rodrigo Salazar, a professor at the Latin American Faculty of Social Sciences in Mexico City. "Society isn't the same. It's a very critical society, a very demanding society, with a strong division of powers."

The final pre-election polls on Wednesday, the last day they could legally be published, showed Pena Nieto with a lead ranging from 8 to 17 percentage points.

Lopez Obrador, 58, was a center-leftist as Mexico City mayor and pioneered some programs that Pena Nieto emulated in the neighboring state of Mexico, such as local pensions for the elderly. But he alienated many voters with his refusal to recognize the narrow victory of National Action's Felipe Calderon in 2006, declaring himself "legitimate president" and mounting protests that gridlocked much of the capital for weeks.

Lopez Obrador says he wants to keep state control over the national oil company, make Mexico self-sufficient in energy and food production, and fund new social spending and jobs programs by cutting waste and corruption, not by raising taxes.

Vazquez Mota, 51, is a former secretary of education and social development in the conservative administrations of President Vicente Fox and his successor, Calderon. She campaigned on the slogan, "different," but has struggled to distinguish herself from Calderon while maintaining the support of the party's power structure.

She has pledged to continue Calderon's war on drug cartels, increase penalties for public corruption and ease rules on hiring and firing employees in order to spur economic growth. On the last day of campaigning, she even promised to make Calderon her attorney-general if elected.

Also running is Gabriel Quadri de la Torre, 57, the candidate of the New Alliance Party, which has links to the powerful teacher's union. His poll support remains in the low single digits.

Mexicans are also electing 500 members of the lower house of Congress and 128 senators. Voters will also select Mexico City's mayor and governors in the states of Chiapas, Guanajuato, Jalisco, Morelos, Tabasco and Yucatan. The president is elected for a single six-year term and cannot stand for re-election. art of that trend