Felipe Calderon, the Mexican president, has blamed drug cartels for the assassination of a front-running candidate for governor in the border state of Tamaulipas.
Calderon said that the assassination on Monday of Rodolfo Torre in northern Mexico shows "organised crime is a permanent threat" and was "trying to interfere in the decisions of citizens and in election processes".
He called on Mexicans to "close ranks to confront it" after local television showed the bodies of Torre and four aides lying on a highway after they were ambushed on their way to a campaign event for the July 4 election.
All five were from the opposition Institutional Revolutionary Party (PRI), which holds power in Tamaulipas state.
Torre's killing is the highest-level political assassination in Mexico since the 1994 murders of PRI presidential candidate Luis Donaldo Colosio in the border city of Tijuana, and prominent PRI party leader Jose Francisco Ruiz Massieu in Mexico City.
Fernando Gomez Mont, the interior minister, condemned the attack as "absolutely reprehensible" and headed to Tamaulipas, across the US border from Texas, to offer his support.
The political killings come after a well-known Mexican singer was killed in a separate incident over the weekend.
Sergio Vega was killed by unknown assassins who intercepted his car in the northern Mexican state of Sinaloa.
Authorities said that the gunmen riddled Vega's car on Sunday with more than 30 bullets near the city of Los Mochis.
Just hours earlier, Vega had given an interview to a Mexican entertainment website in which he said rumours he had been killed were wrong.
Vega was known for singing narcocorridos - songs telling stories about the lives of drug traffickers - and such singers are said to be at risk of being targeted by rival drug gangs.