In the worst corruption scandal of the election, Cancun mayor Gregorio Sanchez was arrested last month for alleged drug trafficking ties, forcing him to drop his campaign for governor of Quintana Roo state. Sanchez was charged with protecting two of Mexico's most brutal drug gangs, allegations he has dismissed as politically motivated.

Investigators work at the scene where Rodolfo Torre Cantu, candidate of the Institutional Revolutionary Party (PRI) for governor of the Mexican state of Tamaulipas, was killed with four aides in an ambush near the airport in Ciudad Victoria, Mexico, June 28, 2010. [Xinhua]

Calderon's government did not say which gang was suspected in Torre's assassination or why he would be targeted.

Tamaulipas, which borders Texas, has become a battleground between the Gulf cartel and its former ally, the Zetas gang of hit men. Gangs have staged bold attacks on security forces, ambushing army patrols and setting up blockades near army garrisons.

Tamaulipas state election authorities met to decide whether to suspend the vote.

The PAN said it would suspend campaigning by its own gubernatorial candidate in Tamaulipas, but PAN party leader Cesar Nava said he hoped the vote would go forward.

Torre, 46, was heading from Ciudad Victoria to the border city of Matamoros to accompany the PRI's mayoral candidates in the closing of their campaigns Monday.

Television footage from the scene showed several vehicles and sheet-covered bodies along the side of the highway.

Torre, 46, held a significant lead in polls as candidate for a coalition comprising two small parties and the PRI, which has long governed Tamaulipas.

"We firmly demand a rapid investigation of these events ... and punishment for those responsible," PRI party leader Beatriz Paredes said in a statement. "Nothing is going to intimidate us."

The PRI, which ruled Mexico for 71 years before losing the presidency in 2000, is hoping that a strong showing in Sunday's elections will put it on the path to regain the presidency in 2012.

The conservative PAN has formed uncomfortable alliances with the leftist Democratic Revolutionary Party to oust the PRI from several states, though not in Tamaulipas. PAN and PRD politicians have insinuated that PRI politicians in Tamaulipas and other states have ties to drug gangs, allegations the PRI dismisses as tired campaign tactics.

Torre, a physician, had served as the state's health secretary from 2005 to 2009. He was married and had three teenage children.