Mexico's political season opened last month in the western state of Michoacan, where the country's former ruling party scored a string of victories seen as a sign of its rising strength nationwide.
The Institutional Revolutionary Party's candidate defeated the sister of Mexico's president in the Michoacan governor's race. It also won a series of local contests, bolstering widespread predictions that this summer the PRI will retake the presidency itself, Mexico's highest office, after 12 years out of power.
But the highly touted victories in Michoacan were in potential jeopardy Wednesday after Mexico's highest electoral court overturned one of them partly because of a strange new form of political advertising _ a party logo emblazoned on a boxer's trunks.
Mexican welterweight Juan Manuel Marquez wore the PRI's green-white-and-red emblem on the left leg of his shorts in his match last month against Filipino Manny Pacquiao in Las Vegas, Nevada.
Marquez lost the Nov. 12 bout, watched live by millions in Mexico. But after the PRI's victories in the next day's elections, the ruling National Action Party complained that the logo had violated a temporary ban on campaigning ahead of the vote, as well as a prohibition on campaigning abroad. It asked for local and statewide races to be overturned.
The federal electoral tribunal ruled in the PAN's favor on Wednesday in the case of the race for mayor of Morelia, the capital of Michoacan.
The court did not immediately release an explanation of the reasoning behind its decision, which requires the Morelia race to be run again on an as-yet-undetermined date over the next five months. But PAN officials said that they were hopeful that the same logic would apply to a pending decision on the governor's race.
The PAN and the leftist Democratic Revolution Party had also complained that Michoacan's powerful drug cartels had intimidated voters on behalf of the PRI. On the eve of the elections, one local cartel placed an advertisement in a newspaper in the town of La Piedad threatening citizens who voted for the PAN.
President Felipe Calderon has said in several public addresses that organized crime helped defeat his sister, Luisa Maria Calderon, and has described drug cartel meddling in elections as a potential national menace.
In addition, the PAN said in its complaint about the Morelia race that winning gubernatorial candidate Fausto Vallejo and Morelia mayoral candidate Wilfrido Lazaro had made unauthorized appearances on television to announce the end of their campaigns.
A high-ranking PAN official, Juan Molinar Horcasitas, said the party had made the same arguments in its appeal of the governor's race, and expected another victory in court.
"According to the law, in the same circumstances the same decision should be taken," he said. "It seems to me that, as a result, the electoral tribunal should annul the governor's race."
A new vote would be highly competitive and almost certain to draw in the national resources of the governing party, the PRI and the PRD in what could become a proxy presidential race.
"The Institutional Revolutionary Party reiterates its conviction that its candidate, Wilfrido Lazaro Medina, obtained a clear, resounding and legitimate victory," the party said in a statement. "The PRI feels that there weren't sufficient grounds to support the court's decision."
Marquez and his managers could not be reached for comment.
Michael Weissenstein contributed.