MANILA, Jan 28, 2024 (BSS/AFP) - Supporters of Philippine President Ferdinand Marcos and his predecessor Rodrigo Duterte were set to hold rival rallies on Sunday, as a battle over the country's constitution highlighted a deepening rift between the powerful clans.

In the nearly two years since Marcos was swept to victory with the help of his vice presidential running mate Sara Duterte -- the former president's daughter -- their so-called "Unity Team" has fractured.

Relations between the families have deteriorated as they seek to shore up their respective support bases and secure key positions ahead of next year's mid-term elections and the 2028 presidential race, which the younger Duterte is widely expected to contest.

In the latest rift, Marcos has backed a campaign for the 1987 constitution, introduced after his dictator father and namesake was ousted from power, to be changed to allow in more foreign investment -- something both Dutertes have publicly opposed.

Critics warn the effort could pave the way for Marcos to get another six-year term, which is currently prohibited.

That would potentially put him on a collision course with the younger Duterte.

"Our constitution is fine... why do we give ourselves a headache when we know all along that this is in order to, you know, you want to stay in power," the elder Duterte said in a recent interview.

Marcos has said the constitution's political provisions, including term limits for public officials, should be tackled later.

On Sunday, Marcos will launch his "Bagong Pilipinas" (New Philippines) campaign in the capital Manila "to usher the nation towards change and development".

The elder Duterte, meanwhile, is expected to attend a separate rally in his southern stronghold of Davao city that has been organised by opponents of constitutional change.

Left-wing political party Akbayan branded the rallies as a "dynastic war among ruling elites" that did not serve the interests of regular Filipinos.

Both events are expected to draw tens of thousands of supporters.

"This would be the most public and deliberate way of letting the people know that the Unity Team is no more," University of the Philippines political science professor Jean Franco told AFP.

- Unravelling alliance -

The alliance between the two families began to unravel soon after the 2022 election, with Marcos giving Duterte the problem-plagued education portfolio instead of her preferred post of defence secretary.

It has gathered pace since then with the powerful House of Representatives Speaker Martin Romualdez, who is Marcos's cousin and also widely expected to seek the presidency in 2028, demoting influential congresswoman Gloria Macapagal Arroyo, a former president and close ally of Sara Duterte.

Romualdez also spearheaded efforts to strip the vice presidency and the education department of millions of dollars in special confidential funds.

And the House, where Marcos has a majority of support, has pushed for a pro-Duterte radio and television broadcaster to be stripped of its licence after one of its hosts made accusations against Romualdez over his travel budget.

House committees also approved resolutions calling on the Marcos administration to cooperate with the International Criminal Court's investigation into Duterte's drug war.

Marcos has said he will not "lift a finger" to help the ICC probe, which he described as "a threat to our sovereignty".