MEXICO CITY -- In a watershed turning point for Mexico, the once-dominant Institutional Revolutionary Party prepared Sunday night to celebrate what a spokesman called the “resounding triumph” of its presidential candidate, Enrique Peña Nieto

Throngs of supporters flocked to the headquarters of the PRI, as the party is known by its Spanish initials, to celebrate the anticipated return to power of a party that kept a monopolistic grip on Mexico for an uninterrupted seven decades.

“Tonight, Enrique Peña Nieto is the next president of … Mexico,” said Luis Videgaray, his campaign manager, citing exit polls that he said showed “a resounding triumph” and an “indisputable” victory by the PRI candidate .

An exit poll from the GEA-ISA polling firm, gave Peña Nieto, a boyish-faced former governor, 42 percent of the vote, compared with 31 percent for his nearest rival, leftist Andres Manuel Lopez Obrador of the Party of the Democratic Revolution.

Josefina Vazquez Mota of the ruling National Action Party and Gabriel Quadri de la Torre of New Alliance also vied for the presidency.

Many Mexicans voted in a sour mood over drug-war violence and an economy only recently gaining steam. They were ready to give a new chance to a party that ruled from 1929 until 2000, casting aside concerns over its autocratic past.

Scattered irregularities were reported at a few of the nation’s 143,000 voting stations, but for the most part the vote appeared calm and orderly.

In a closely watched race in the capital, exit polls gave a landslide victory to the Party of the Democratic Revolution’s Miguel Angel Mancera, allowing the party to hang on to Mexico City Hall, which it has controlled for a decade and a half.

It is an election that has been closely watched in the United States. Not only does the United States share a nearly 2,000 mile border with Mexico, but the two countries have key mutual interests in areas such as trade, energy, homeland security and migration. Mexico is the third largest U.S. trading partner (after China and Canada) and also a vital source of crude oil to U.S. markets.

As many as six million U.S. jobs depend on trade with Mexico and 10 million U.S. citizens visit Mexico every year. Increasingly, criminal gangs in Mexico have tentacles that reach into U.S. cities.

The apparent return of the PRI is an about-face for Mexico. Mexicans fought for decades to topple the PRI from its 71-year grip on power, eventually ousting the party in 2000.

Some critics say a PRI triumph would lurch Mexico backward to its authoritarian past. Supporters say the PRI has learned from its past and won’t rule as it once did, even if some within its ranks do not embrace the change.

The party would face opposition in Congress, a robust media, a largely independent Supreme Court and strengthened civil society groups that employ social media tools aggressively.

“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.

In a sign of the forces that will keep an eye on the PRI, when the PRI’s president, Pedro Joaquin Coldwell, entered a polling station and cut in line, chants and shouts of “Corrupto!” rang in the air from angry voters. A witness shot the scene with a cellular telephone and the video quickly splashed around the Internet.