Pamela Teran, a candidate from the ruling Institutional Revolutionary Party (PRI) in Juchitan in Oaxaca state, was killed along with photographer Maria del Sol Cruz and another person as they left a restaurant.
Her father is Juan Teran, identified as a drug cartel leader in her area. He has been jailed since 2017 on organised crime charges.
Cruz worked on indigenous affairs with the Oaxaca state government. The third person killed was believed to have been a bodyguard.
So far this year, more than 30 politicians have been murdered in Mexico.
Jose Manuel Lopez Obrador, who is making his third bid for the presidency, looks increasingly hard to beat in the single-round vote -- amid frustration with festering corruption, record levels of violent crime and a lacklustre economy.
A recent poll found ruling party candidate Jose Antonio Meade would get 19 percent of the vote, and independent Jaime Rodriguez 3 percent.
Pamela Teran was a candidate for the ruling Institutional Revolutionary Party. Source: Twitter
