Mexico will hold its presidential election Sunday, and most polls suggest the Institutional Revolutionary Party, or PRI, is expected to win after being ousted in 2000.

But the party is embroiled in a scandal involving the country's largest television network. There are accusations right-wing frontfrunner Enrique Peña Nieto has an unfair advantage in the race because Televisa has been promoting him for years.

Mexican presidents are barred by the constitution from running for a second term — they're limited to one six-year term — but Josefina Vázquez Mota, the candidate for outgoing President Felipe Calderon's ruling conservative National Action Party (PAN), is in third place, according to most polls.

Vázquez Mota is an economist and the first woman to run for the country's presidency.

Andres Manuel López Obrador, candidate for the left-leaning Party of the Democratic Revolution and a former mayor of Mexico City, is said to be in second place. He narrowly lost to Calderon in the 2006 election.

CBC's Keith Boag, in Mexico City, talked to Denise Dresser, a political analyst and faculty member at the Instituto Tecnológico Autónomo de Mexico (ITAM), about how Calderon tried to take on the drug cartels and failed, how his  presidency "was trapped by the war on drugs."

The Calderon administration used the military in "an effort to combat the rising violence, the executions, the territorial control that many cartels established in the country," Dresser said.

In retrospect what he did was "kick a hornet's nest," and violence actually rose under his term, she said.

"They only multiplied when the heads of the cartel were apprehended the groups dispersed and much more infighting occurred.

"The homicide rate doubled in 2008, doubled again in 2009, and we've ended up with 60,000 people dead in the last term of Felipe Calderon. So many people view the war on drugs as a failure, as something that was perhaps intended and carried out with good intentions, but very badly executed," Dresser said.

Two days before the election, an explosive device blew up inside a truck parked outside the town hall of Mexico's northern city of Nuevo Laredo, injuring seven people but causing no fatalities, officials said.

Police did not immediately confirm whether Friday's attack was drug-related but Mexican cartels have been increasingly using improvised explosive devices in cars. Most are relatively small compared to the car bombs used in conflict zones such as Iraq.

The explosion went off mid-morning, tearing apart the vehicle and damaging 11 other cars as well as a wall of the town hall, the Tamaulipas state attorney general's office said in a statement.

With files from The Associated Press