Gunmen have assassinated the front-running candidate for governor of a Mexican border state in what the country's president Felipe Calderon called an attempt by drug gangs to sway local and state elections.
The assailants ambushed Rodolfo Torre's vehicle as he headed to the airport in Ciudad Victoria, capital of Tamaulipas, a state torn by a turf battle between two rival drug cartels. At least four other people travelling with him were killed.
Mr Calderon said that the events proved "organised crime is a permanent threat and that we should close ranks to confront it and avoid more actions like the cowardly assassination that today has shaken the country".
He added: "We cannot and should not permit crime to impose its will or its perverse rules."
He warned that organized crime "wants to interfere in the decisions of citizens and in electoral processes."
Mr Torre, of Institutional Revolutionary Party, or PRI, is the first gubernatorial candidate assassinated in Mexico in recent memory. He is the highest-ranking candidate killed since Luis Donaldo Colosio, also for the PRI, was gunned down while running for president in 1994.
The attack was the biggest setback yet for Sunday's elections in 12 states. Corruption scandals, threats and attacks on politicians have raised fears for months that Mexico's powerful drug cartels are buying off candidates they support and intimidating those they oppose.
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 military patrols and setting up blockades near army garrisons.
Last month, gunmen killed Jose Guajardo Varela, a candidate for mayor of the Tamaulipas town of Valle Hermoso. Guajardo, of Calderon's National Action Party, or PAN, had received warnings to drop his campaign.