It looks almost inevitable now that the Philippines' next president will be a man who has said he would kill his own children if they took drugs.

Rodrigo Duterte has a commanding lead in opinion polls ahead of the May 9 election, much to the dismay of business chiefs, investors and diplomats who see him as a clown with no clear economic policies and a possible liability for the country.

Brash and openly scathing of the political establishment on the campaign trail, he has been likened to US presidential candidate Donald Trump.

And yet, Duterte's supporters point to his past to argue that as president, he would not be the disaster that critics fear.

As mayor of Davao city in the south of the country, they say, the 71-year-old former prosecutor waged war on drugs, crushed crime rates, lifted investment levels, improved health and education, and delegated policy issues to technocrats.

"He communicates in a very rough way, but he has a policy group, he consults, he listens, there's mitigation after his speeches," said Jesus Dureza, a former congressman who went to school with Duterte and is now involved in his campaign.

Eurasia Group, a global risk research firm, said in a report this week that the Philippines is likely to continue on the pro-growth and reform-oriented path set by outgoing President Benigno Aquino, regardless of who wins the election.

It said that while Grace Poe, a senator who was at one stage the front-runner in opinion polls, appears more engaged and knowledgeable on economic policy, Duterte defers to capable advisers and is likely to pursue infrastructure development, administrative streamlining and openness to foreign investment.

Last week, members of the country's leading business club were stunned when Duterte addressed them in his usual style, threatening to kill criminals and making risque comments about Viagra use and his sex life, but shedding little light on the economic policies he would pursue if he became president.