A tough-talking Philippine presidential candidate has vowed to kill his children if they ever take drugs, as analysts warn his scandalous remarks about raping a missionary could hurt his election chances.

Rodrigo Duterte has become the surprise favourite to succeed President Benigno Aquino after making a name for himself in a southern Philippine city infamous for vigilante death squads.

The 71-year-old mayor of Davao has promised to crack down on the Philippines' criminals, and on Sunday he vowed to be a harsh president if elected next month.

"I am really angry," he said in the final televised debate before the May 9 polls.

"They say I am a killer. Maybe I am."

Asked what he would do if he learned one of his children was involved in drugs, Mr Duterte replied: "I will kill him."

Analysts say his profanity-laced campaign has resonated in a chaotic, high-crime society with limited opportunities for a vast underclass working for a tiny elite.

A poll by Pulse Asia, released on Sunday, showed Mr Duterte pulling ahead of his rivals.

Missionary joke could be costly: analysts

But he caused outrage with a recent joke about the jailhouse rape of an Australian lay Christian missionary who was killed in a 1989 prison riot in Mr Duterte's own city.

He then threatened to sever ties with allies the United States and Australia after their ambassadors criticised his remarks.

Ramon Casiple, executive director of the Institute for Political and Electoral Reform, said Mr Duterte's joke could cost him the elections.

"The rape issue is personal. It touches on family values," Mr Casiple said.

"I talked to some of his hardcore followers, some of them have already turned away from him."

Ronald Holmes, head of research institute Pulse Asia, said the furore broke out too late to affect its weekend survey but could affect the final vote.

AFP