Speaking in the heart of Rodrigo Duterte's stronghold Mindanao ahead of May mid-term elections, Marcos blasted the former president -- without mentioning him by name -- for everything from his deadly drug war to alleged fraud during the Covid-19 pandemic.

But it was his country's increasingly fraught relationship with China that represented a fresh line of attack.

"You will choose if we will go back to the time when our leaders wanted the Philippines to be a province of China," he told a rally in Davao del Norte province.

"None of (our candidates) are Chinese lackeys who cheered while our Coast Guard was being water-cannoned by big Chinese ships," he said.

Chinese vessels have engaged in a series of high-profile confrontations with Philippine ships in disputed waters of the South China Sea claimed by Beijing despite an international ruling that their assertion has no legal basis.

"I hope we won't go back to the past, when our leaders were pimping our country as a gambling centre for foreigners," Marcos said in a reference to now-banned offshore gambling sites often run as scam centres by Chinese nationals.

Dennis Coronacion, head of the political science department at Manila's University of Santo Tomas, said making China a talking point was a potentially winning formula.

"In the past, foreign policy and the talk about China was never an election issue, so if it's going to be... I think he can get the support of many Filipinos," he said, adding surveys showed citizens felt strongly about it.

"They don't like what China is doing to us."

University of the Philippines political science professor Jean Franco said Marcos was underscoring a very clear difference in how the two administrations had approached the China issue.

"It should be an election issue given that it's an issue that has to do with our sovereignty," she said. Marcos could be "banking on the fact" that clashes in the South China Sea are widely covered by the media, Franco said.

Marcos is widely seen as having aggressively pushed back against Chinese claims, a change from Duterte's cosier relationship with Beijing.

The battle between the two political dynasties is at the heart of the mid-term race, which could determine the political future of impeached Vice President Sara Duterte, who has fallen out spectacularly with Marcos.

Rodrigo Duterte's eldest daughter, she now faces a Senate trial on charges of "violation of the constitution, betrayal of public trust, graft and corruption, and other high crimes" that could bar her from public office.

Half of the upper house body's 24 seats are up for grabs in May and its make-up could play a key role in the trial's outcome.