MEXICANS HAVE ALWAYS ACCEPTED that life demands that they embrace the starkest contradictions, the late author and poet Octavio Paz once said. As the country’s presidential election campaign nears its conclusion, Mexicans have much to embrace.

Consider the following Paz-like contradictions. Mexico’s growth rate has lagged those of the BRIC nations, millions of its citizens are jobless, and millions more work illegally across the border in the U.S., yet the country has become one of the world’s great manufacturing exporters, shipping more flat-panel TV screens abroad than South Korea. A five-year, drug-related war has left more than 50,000 Mexicans dead, many of them slaughtered brutally, their torsos scattered on public highways, yet the country last year drew record numbers of foreign tourists.

In this country of contradictions, perhaps the ultimate irony is the Institutional Revolutionary Party. The PRI (its Mexican acronym) is one of the most corrupt ruling parties in Latin American history. Its leaders stole untold billions of pesos, rigged hundreds of elections and muzzled or bribed opponents to rule for most of the 20th century — almost as long as the Soviet Union’s Communist Party — before public disgust finally drove it from power and into disgrace a dozen years ago. Yet today the largely unreformed party is on the verge of winning a resounding victory in July’s presidential and congressional elections.

The party’s presidential standard-bearer is Enrique Peña Nieto, a PRI princeling who combines the deft political skills of a party insider with the youthful....