Mexico’s former ruling party, promising to put the country back in the big leagues of emerging economies, won the presidency Sunday, capping a remarkable comeback for a party once called “the perfect dictatorship” for its long grip on power.

Enrique Peña Nieto, a telegenic former governor of the Institutional Revolutionary Party, or PRI, won with about 38% of the vote versus 31% for his closest challenger, leftist Andrés Manuel López Obrador, according to a partial vote count by Mexico’s election agency.

Josefina Vázquez Mota, Mexico’s first major female presidential candidate and a member of President Felipe Calderón’s National Action Party, or PAN, trailed with 26%.

The final official result might vary slightly, election officials said.

Ms. Vázquez Mota conceded defeat, but Mr. López Obrador said he would wait for final results in the coming days to decide what to do. Associates said he would likely contest the results in court, alleging that the PRI broke campaign spending limits and had favorable coverage in the media.

Mr. López Obrador, a former Mexico City mayor, narrowly lost the 2006 presidential election and refused to acknowledge defeat, leading to months of street protests. But rather than rail about a vast conspiracy, as he did in 2006, he sounded more subdued this time around, talking about a “lack of equity.”

The return of the PRI, which ruled Mexico for 71 years through an extensive patronage system that Peruvian writer Mario Vargas Llosa dubbed “the perfect dictatorship,” marks a stunning comeback for a party that nearly fell apart after it lost its first presidential election in 2000. After a third-place showing in 2006, the party has united around its new face: Mr. Peña Nieto, a 45-year-old former state governor.

Even in victory, the party was supported by only four in 10 Mexicans. In his victory speech, Mr. Peña Nieto told cheering supporters that the PRI had been given a second chance at power, and must show voters that it can govern better than in the past, when it was dogged by corruption scandals. “We have to show that we understand Mexico has changed,” Mr. Peña Nieto said.

After back-to-back conservative PAN administrations, many voters said they were fed up with Mexico’s sluggish economic growth, which has averaged just 2% a year since 2000. During that time, Mexico has been eclipsed by other rising stars like China and Brazil.

Mexico has also been plagued by a wave of drug-related violence that has killed some 55,000 people in the past six years and hampered economic growth. Last week, for instance, corrupt federal police gunned down three other police in Mexico City’s international airport, sending frightened passengers ducking for cover.

Thanks to slow growth and the drug war, the PAN was dealt its worst election result since 1988, falling to third place and losing two governorships in its former strongholds of Jalisco and Morelos.

Mr. Peña Nieto praised Mr. Calderón’s decision to take on warring drug cartels, and vowed to continue the fight even as he shifts the focus to reducing violence rather than capturing kingpins.

He tried to ease worries that the PRI might try to strike a bargain with cartels to reduce violence in exchange for gangs being left alone to ferry drugs. “Let me be very clear: Confronted with organized crime, there will be no pact and no truce.”

The race shaped up as a contest of who could get Mexico moving again. Many here say Mexico grew complacent after becoming the first developing economy to sign a free-trade deal with an advanced economy in the North American Free Trade Agreement, which linked the economies of the U.S., Mexico and Canada in 1994.

During the campaign, the PRI billed itself as “responsible change,” safer than Mr. López Obrador’s populist PRD, and more capable than the PAN, which struggled to get major initiatives through Mexico’s divided congress. Many of those initiatives were blocked by the PRI itself.

Mr. Peña Nieto has promised to take on the very economic changes that have eluded past presidents, including opening Mexico’s closed energy sector to private investment, overhauling the tax code to lift paltry government revenue and rewriting labor laws to make it easier to hire and fire workers.

In his speech, he laid out a bold vision of reform, stressing the need to carry out meaningful changes. “It’s time to transition from the country we are to the one we could be and deserve,” he said.

Progress in ending Mexico’s stalled reform agenda, especially in opening its oil industry, is likely to create a big impression on foreign investors.

“Any move in the energy sector in Mexico begins to change the Mexican narrative,” said Gray Newman, chief economist for Latin America at Morgan Stanley. Investors are ready to give Mexico another chance at a time when China and other big emerging economies are slowing down, he said.

But change will be hard to come by. The PRI will face a divided congress, and the party will need support from the PAN for any constitutional overhaul like opening the energy sector, a state monopoly since 1938. The last Mexican president from the PRI, Ernesto Zedillo, also had trouble passing major legislation when the party lost control of congress in 1997.

Historian Enrique Krauze said he expected the PAN to go along with the free-market overhauls, overcoming the temptation to get even with the PRI for having blocked PAN initiatives. But even then, he said, the efforts at reform will be challenged within the PRI itself and by Mr. López Obrador, who can summon his followers to the streets.

Mr. Peña Nieto and his advisers also “will face the other tribes in the PRI—and these aren’t just tribes, they are dinosaurs.…The skills of the younger generation will be tested,” he said.

—Laurence Iliff, Juan Montes and Jean Guerrero contributed to this article.

Source: Wall Street Journal