Mexico City -- On the cusp of a likely historic shift, Mexican voters went to the polls Sunday widely expected to elect Enrique Pena Nieto to the presidency and return the once-dominant Institutional Revolutionary Party to power.

Many voters cast ballots in a sour mood over drug war violence and an economy only recently gaining steam, poised to give a new chance to a party that ruled during seven uninterrupted decades, trusting it has abandoned its authoritarian and often corrupt ways.

Early reports said the vast majority of the nation’s 143,000 voting stations opened normally, without interference. Long lines snaked out of precincts in the capital.

Pena Nieto, a boyish-faced 45-year-old former governor representing the PRI, as the party is known by its Spanish initials, faced Andres Manuel Lopez Obrador of the leftist Party of the Democratic Revolution, Josefina Vazquez Mota of the ruling National Action Party and Gabriel Quadri de la Torre of New Alliance.

Mexicans fought for decades to topple the PRI from its 71-year monopolistic grip on power, eventually ousting the party in 2000.

Some critics say a triumph of the PRI would lurch Mexico backward to its authoritarian past. Others suggest that Mexico has changed and the PRI can’t rule as it once did, even if some within its ranks sought to.

The party would face opposition in Congress, a rambunctious media, a largely independent Supreme Court and strengthened civil society groups.

“Mexico is more globalized that it was 12 years ago. That means a party can’t do the same things it did 12 years ago,” Juan Rafael Aguilar, an unemployed business administrator, said after voting in the capital’s Magdalena Contreras district.

Many Mexicans feel frustration at the past 12 years of rule by the center-right National Action Party (PAN), which failed to usher in wholesale reforms of a PRI-designed political system.

“At the end, the lasting impression is of enormous wasted opportunity,” Jorge Zepeda Patterson, a political scientist and columnist, wrote in the El Universal newspaper Sunday.

Under President Felipe Calderon, the party brought U.S.-Mexico security cooperation to unprecedented levels. But while deploying soldiers to the streets, and capturing numerous drug barons, it failed to rein in runaway killings and rampant violence that have left pockets of the country under control of gangsters.

Calderon treats the toll from crime-related killings as a state secret, wary that the bloodletting will stain his legacy. Outside experts say they believe the toll has surpassed 55,000 deaths since late 2006.

Perhaps because the PRI seemed on the threshold of returning to power, the election had the feel of a watershed, a major redrawing of the political map.

In addition to the presidency, Mexico’s 79.4 million voters were also replacing all 128 senators and 500 members of the lower-house Chamber of Deputies, as well as more than 1,400 state and local officials. Experts said the PRI would obtain a relative majority in both houses, and maybe even an absolute majority in one.

Mexicans were also voting for governors of six states and the mayor of Mexico City, a position with powers equal to a governor and widely considered the second most important political post in the country after the president. City Hall was expected to remain in the hands of the leftist Party of the Democratic Revolution.