Facing widespread dismay over the assassination of a leading gubernatorial candidate, President Felipe Calderon on Tuesday urged fellow Mexicans to join hands against the forces of organized crime that he said were to blame.

The killing of Rodolfo Torre on Monday in northern Mexico has added to Calderon's political headaches as voters are to head to the polls Sunday in 14 states to pick a dozen governors and hundreds of mayors and lawmakers.

"United, Mexicans can and will overcome a common enemy that today threatens to destroy not only our tranquillity but our democratic institutions," Calderon said in a broadcast message. "It's in the divisions between Mexicans where criminals find spaces and vulnerabilities to harm Mexico."

Torre was leading polls in the border state of Tamaulipas. His death in a highway ambush, which also killed four members of his campaign team, was the latest sign of a deteriorating security situation as Calderon pursues an offensive against drug traffickers.

Drug-related violence has left more than 23,000 people dead since Calderon, of the conservative National Action Party, or PAN, launched his anti-crime offensive upon taking office in late 2006. Amid the rising carnage, he recently has delivered lengthy messages to convince Mexicans that the crackdown is needed to save the nation from the grip of criminal gangs.

Calderon appeared to have political concerns in mind Tuesday as he urged critics in rival parties to join in a "frank, respectful and constructive" discussion aimed at forging a shared crime strategy.

"In the face of the challenge that organized crime poses today, there is no margin for seeking political dividends," he said.

As the death toll has risen, Calderon's rivals in the Institutional Revolutionary Party, or PRI, and left-leaning Democratic Revolution Party have increasingly assailed the government's crackdown as ineffective.

In Tamaulipas, governors and officials with the PRI, Torre's party, joined hundreds of other mourners at a memorial service in the state capital, Ciudad Victoria.

PRI officials had yet to name a replacement candidate. Some polls had shown Torre having a lead of 30 percentage points or more over Jose Julian Sacramento Garza of the PAN.

Officials in Tamaulipas said elections in the state — to pick the governor, 43 mayors and 36 state legislators — would be held Sunday as planned.

The state is a PRI stronghold with a long history of suspected collusion between elected politicians and traffickers. For months, Tamaulipas has been the setting for a bloody battle between the Gulf cartel and former allies known as the Zetas.

Calderon blamed organized crime gangs for the assassination, but did not name a specific group.

The slaying set off shock waves, prompting some commentators to ask whether it might stir a largely passive society into concerted action against crime. Others saw it as a sign both of how deeply criminal groups had infiltrated Mexico's political system through bribery and intimidation and of how much they wanted still more influence.

"Organized crime has voted. It imposed its will by means of violence," said an unusual editorial in the daily Reforma newspaper, run on the front page. "It stole from the people the ability to elect."