MEXICO CITY — The apparent victor of Mexico’s presidential race, Enrique Pena Nieto, struggled Monday with the sticky bonds of his party’s notorious past, the limitation of his election mandate and an opponent who refused to concede defeat.

His long-ruling and now-returned Institutional Revolutionary Party, the PRI, won only about 38 percent of the vote and is unlikely to get a majority in Congress.

He faces an old guard in the PRI that still exercises considerable power, a war with fierce drug cartels and a still sluggish economy. His closest rival, leftist Andres Manuel Lopez Obrador, who polled a higher-than-expected vote of about 32 percent, refused to accept the loss.

President Barack Obama called Pena Nieto on Monday to congratulate him. The U.S. Embassy in Mexico City said Obama told him the United States “looks forward to advancing common goals, including promoting democracy, economic prosperity, and security in the region and around the globe, in the coming years.”

Pena Nieto’s account of the talk suggested his party has left behind the touchy nationalism of the past. He expressed interest in cooperation in security, commerce and infrastructure, but didn’t bring up the traditional Mexican issue of U.S. immigration reform.

In Sunday’s elections, Mexicans voted above all for a known quantity, the camera-friendly candidate of the party that ruled Mexico without interruption from 1929 to 2000.

But the PRI returns to power in unknown political terrain, where Mexico is more divided, more violent and less tightly controlled, raising the potential for political disputes on top of the drug war. The battle against drug cartels has already cost more than 47,500 lives and may have contributed to the decline of President Felipe Calderon’s conservative National Action Party.