MEXICO CITY — A review of Mexico's closest presidential race in history today is a key test for an electoral system that helped reverse decades of ballot-rigging and has been championed as an example for emerging democracies.

A preliminary count of Sunday's election showed Felipe Calderon, of the conservative ruling National Action Party, leading by 1 percentage point over leftist Andres Manuel Lopez Obrador of the Democratic Revolutionary Party.

Because of the closeness of the race, however, the Federal Electoral Institute won't declare a winner until completing a review required by law that begins today and could take days.

The count will be crucial to proving the election was clean in a nation that emerged only six years ago from 71 years of one-party rule sustained by widespread vote-rigging.

"Such a close race is a nightmare scenario," said Ted Lewis, an election observer for the San Francisco-based Global Exchange. "If the ruling party wins by a hair, a lot of people will jump to the conclusion that something is amiss."

Lopez Obrador, a former Mexico City mayor with fervent support among the poor, has claimed the preliminary count was manipulated and his party has demanded a ballot-by-ballot recount. His claims have stoked fears of violent protests.

It was unclear if electoral officials would meet the party's demands. Mexican law allows for a manual recount only if the sealed packages of ballots have been tampered with, or if the tallies are obviously miscalculated, wrongly filled out or illegible.

Luis Carlos Ugalde, president of the electoral institute, suggested there would not be a major recount. "In exceptional cases, you can open the electoral package but ... the votes were already counted Sunday in the presence of the parties."