“The presidency that I want to enact and contribute to is modern, open to criticism, ready to listen and to work with all forms of political expression,” he said at a news conference here.

 Standing in front of a banner that said “Mexico won,” he also said that he had spoken to President Obama this morning, and they had discussed how to “expand the relationship” between Mexico and the United States, especially in terms of economic integration. 

His comments —   in which he also reiterated that his government would make no pact or agreement with drug cartels —   seemed designed to assuage fears in Mexico and the United States about both Mr. Peña and his party, the Institutional Revolutionary Party or PRI,  which vaulted back to power on Sunday after ruling Mexico for decades with an autocratic grip. Arguing that he understood the changes that have come to Mexico since the PRI was booted from the presidency in 2000, Mr. Peña said he hoped to create “a democratic presidency that governs for all Mexicans.”

 Preliminary results showed a clear, if not quite resounding victory. With 96 percent of the vote counted, Mr. Peña had won 38 percent of the vote, defeating both the incumbent conservative party and the candidate who nearly beat the conservatives last time. But Mexican voters also seemed hesitant to give the party total control: the PRI-led coalition in congress —   while still the largest in Congress — does not appear to have won an outright victory, according to the preliminary returns. Mr. Peña’s apparent margin of victory —   hovering at about 6 percent — also looked to be about half what most polls suggested before election day, with the leftist party enjoying a late surge.

 “It was a good night for the PRI because they recaptured the presidency but it was not the devastating knockout punch some predicted,” said Eric Olson, an analyst with theWoodrow Wilson International Center for Scholars. ¨They will do well to keep that in mind as they begin to form their government and rule.”

A “quick count” based on a sampling of returns from across the country, announced by election officials late Sunday night, showed Mr. Peña Nieto with 38 to 39 percent of the vote and a 7-point lead over Andrés Manuel López Obrador, a former Mexico City mayor who lost narrowly in 2006 and is a member of the left-leaning Party of the Democratic Revolution.

By Monday morning,  Mr. Peña Nietos’s lead had narrowed slightly, to around 5 percentage points, and  Mr. Lopez Obrador had not yet conceded.

He said late Sunday night that he would wait for complete vote tallies in the coming days. ¨The final word has not been said,¨he told supporters.

In 2006 Mr. Lopez Obrador refused for 48 days to accept defeat and led street demonstrations demanding a full recount.

The conservative candidate, Josefina Vázquez Mota, a former cabinet secretary who sought to become Mexico’s first woman president, was running third with 25 to 26 percent. Earlier in the evening, exit polls released by several news organizations pointed to similar results, though with a somewhat wider lead for Mr. Peña Nieto.

He addressed supporters shortly before midnight, repeatedly saying that “Mexico won,” promising to govern openly and with accountability, and trying to knock down any suggestion that he would reach accommodations with criminal groups, as his party has been accused of doing in the past.

“There will be no deals or truce with organized crime,” he said.

Though Mr. Peña Nieto was declared the winner and President Felipe Calderón telephoned to congratulate him, the preliminary results suggested that he had not won an unequivocal mandate, garnering fewer than half the total votes and trailing in some of the most violence-plagued states.

Ms. Vázquez Mota made a concessionary speech soon after the polls closed. Without mentioning Mr. Peña Nieto directly, she alluded to concerns about the PRI, saying she would continue to fight against “the return of authoritarianism, of corrupt rule, impunity and capitulating to organized crime.”

Mr. Peña Nieto, who has visited Washington and New York several times in the past year to introduce himself to lawmakers and opinion leaders, has promised more efficient, expanded trade with the United States and predicted that relations would be strong. But Washington will be watching closely for any hint of his administration’s easing the pressure on drug traffickers.

In his campaign, he promised to refocus the drug war more on combating the violence afflicting Mexicans, and hardly mentioned attacking drug trafficking itself or taking down cartel leaders.

That stance might have been meant to distance him from Mr. Calderón, whose militarized approach to the drug war left many Mexicans uneasy over the heavy civilian toll — more than 50,000 drug war-related deaths in recent years — and eager for a better strategy. Mr. Calderón acknowledged Mr. Peña Nieto’s victory in remarks late Sunday night.

That some in Washington intend to watch Mr. Peña Nieto closely on the drug war issue was demonstrated minutes after the preliminary results were announced, when Representative Michael McCaul, Republican of Texas, issued a statement that combined congratulations with a warning.

“I am hopeful that he will not return to the PRI party of the past, which was corrupt and had a history of turning a blind eye to the drug cartels,” Mr. McCaul said of Mr. Peña Nieto in the statement.  

No single issue dominated the campaign — not the drug war or the economy, which is growing but leaving the poor behind and lagging in raising wages.

Instead, polls indicated that voters felt a general malaise and fatigue after 12 years of rule by the conservative National Action Party, and disenchantment with Mr. Calderón, whose six-year term ends in December. Mexican presidents are limited by law to a single term in office.

Mr. Peña Nieto, 45, a former governor of Mexico State, which nearly surrounds Mexico City and is the nation’s most populous, rose rapidly from obscurity through his party’s ranks in the state, where relatives of his had played prominent roles.

Critics suspected the party’s old guard was hiding behind his fresh face, and in an interview last month he staunchly defended the party, saying that opponents focused too much on its misdeeds and overlooked the economic growth and social programs the party fostered in its heyday.

He campaigned without an ideological bent, as he had when running for governor, and presented himself as a pragmatic manager, publicly signing pledges to get things done.

He made the economy his centerpiece, saying that he would create jobs and lift wages, and that he planned to shore up Pemex, the country’s national oil monopoly and a vital source of public revenue, by opening the company to private investment.

Mr. Peña Nieto took a lead in the polls that he maintained even as his opponents attacked his party, suggesting that it would try to bring peace in the streets by making deals with the drug networks and would rule by corruption and patronage, as it had in the past.

University students held demonstrations against him — some protesters briefly blocked his path as he tried to vote Sunday — and accused the major television channels of favoring him with biased coverage, especially Televisa, the largest network. Mr. Peña Nieto appeared frequently on Televisa in news coverage, advertisements and infomercials, and is married to one of the network’s most popular soap opera stars, Angélica Rivera, who campaigned prominently for him.

The atmosphere at PRI headquarters Sunday night was jubilant. Supporters wearing the party’s signature red T-shirts waved Mexican and party flags as reggaeton and cumbia music pounded on speakers, and cheers erupted with every new bit of news. In addition to the presidency, the exit polls indicated that the party may have made gains in the federal legislature and picked up control of at least two more states, bringing its total to 22 out of 31.

Some voters said on Sunday that they favored the PRI out of tradition, or a sense of nostalgia for what they remembered as a more stable time.

“Better the old one you know than the new to get to know,” Jorge Osorio, 70, said, recalling the words of his grandfather as he voted near Mexico City for the PRI candidate.

Still, such is the taint of the PRI, which kept power for decades through rigged elections, corruption and patronage, that many people were reluctant to admit on Sunday that they supported Mr. Peña Nieto. Leonor Acosta Chavira, 77, who waited to vote with about 300 people in Tijuana, said she was worried that others would get angry if they knew of her choice.

“I vote for the party, no matter the candidate,” she said.

Many others declined to divulge their choice because of government admonishments for a “voto secreto.” But some said they could not stomach giving the PRI another chance.

“The PRI had its opportunity for 70 years, and the country didn’t progress,” said Moisés Basilio, 29, of Guerrero State, a supporter of Mr. López Obrador. “We’re a country with great potential. The problem is the government has never made the people a priority.”

Liliana Patiño, 33, a voter in Ciudad Nezahualcóyotl, outside Mexico City, said she backed Ms. Vázquez Mota, the candidate of the incumbent party, because she “wanted to give them more time to complete what they started.”

Nearly 80 million Mexicans were eligible to vote, with turnout estimated at 62 percent, slightly higher than in 2006. Federal election authorities said the voting had gone smoothly despite some bumps, and a group of observers from the Organization of American States said there had been few problems.

At some polling stations, there were reports of a shortage of ballots and missing paperwork, and watchdog groups were collecting reports of vote-buying and other irregularities. Social media sites flared with depictions of disorganization and delays at some polling stations. In one video circulated on YouTube and Twitter, the chairman of the PRI was seen cutting ahead in line to vote, to a chorus of catcalls of “Corrupt!”

A government hot line set up to receive reports of fraud had received more than 1,300 calls by midafternoon, but it was unclear whether the problems were any more numerous than in previous elections, or had merely been documented by more voters with smartphones.

There were also reports of security problems at some sites, but government officials did not give specifics; they said that over all, feared drug-related violence had not come about.

Mr. Osorio, the PRI supporter, said his name had already been crossed off a list of voters when he arrived at his polling station. Election workers decided it was an error and let him vote.

All the parties have been accused of giving voters money and goods in exchange for support, but the PRI has a reputation for being particularly adept at it.

Eber Alvarez, 23, a dental student in Ciudad Nezahualcóyotl, said 15 of his relatives and friends had been paid up to 500 pesos each by the PRI, while the Party of the Democratic Revolution and the National Action Party never seemed to offer more than 400, and typically less. He said he did not receive anything himself, but registered a small protest by drawing a giant X across his ballot. “I annulled it,” he said. “The system is a joke.”

During the PRI’s rule, its leaders were credited with modernizing the economy and creating lasting social programs — the poet Octavio Paz called them “philanthropic ogres” — but they stifled political dissent and rigged elections, and were eventually undone by economic crises and corruption scandals.

The National Action candidate, Ms. Vázquez Mota, 51, struggled to find a message and ran a gaffe-prone campaign that sank her into third place. Mr. López Obrador, 58, hoped to capture the anti-PRI vote with blistering attacks on the party and on Mr. Peña Nieto, and made a point of campaigning in PRI-run states.

Still, Amado Domínguez Hernández, 43, who lives in Ciudad Juárez, across the border from El Paso and one of the Mexican cities most affected by drug violence, said he had chosen Mr. Peña Nieto in the end because he believed Mr. Peña Nieto would improve security.

“Honestly, I think he is going to change the country and he’s going to change it for the better,” he said.