Mexican television networks and newspapers are reporting that a front-running candidate for governor in northern Tamaulipas state has been shot to death.
Government spokesmen say they cannot confirm the reports immediately.
The federal Public Safety Department is dispatching police to the Gulf coast state to investigate.
Candidate Rodolfo Torre of the Institutional Revolutionary Party was reportedly attacked on a highway Monday.
It would be the first killing of a gubernatorial candidate in recent Mexican history.
Presidential candidate Luis Donaldo Colosio was assassinated in 1994.
He was also from the Revolutionary Party, known as the PRI.
Mexicans vote for 12 governorships on Sunday.