Philippine President Rodrigo Duterte’s party on March 22 threw its support behind presidential election favourite Ferdinand Marcos Junior, as his main rival appears to gain momentum in the race for the top job.

Voter surveys show the son and namesake of the country’s former dictator heading towards a landslide victory in the May 9 polls, fuelled by a massive social media campaign pumping out positive misinformation about him.

Marcos Jr, 64, has teamed up with vice presidential frontrunner and first daughter Sara Duterte, in a formidable alliance between the scions of two of the country’s most powerful political families.

“We are endorsing the candidacy of Bongbong Marcos Jr – this is a party decision,” said Alfonso Cusi, president of PDP-Laban and leader of the pro-Duterte faction of the party.

Cusi would not confirm if the decision meant Duterte also supported Marcos Jr, who he has previously described as “weak”.

Presidential spokesman Martin Andanar said Duterte’s position was “not clear”.

Outgoing presidents usually identify their preferred successor, who they hope will shield them from criminal charges brought by rivals as much as protect their legacy.

They rarely end up winning.

Duterte, who is constitutionally barred from seeking a second six-year term and remains popular among many Filipinos, had chosen loyal aide Senator Christopher Go to replace him, but he pulled out of the contest.

Since then, Duterte has been cryptic about who he wants to see in the presidential palace.

He recently said the next leader should be a lawyer and compassionate – raising eyebrows among observers as the description seemed to best fit his nemesis, incumbent Vice President Leni Robredo, who is a distant second behind Marcos Jr in the polls.

The positions of president and vice president are elected separately in the Philippines.

But University of the Philippines political science professor Jean Franco said PDP-Laban’s backing was “tantamount to Duterte endorsing the candidacies of Marcos and his daughter”.

“It’s big,” she said, adding it gives “local officials the signal that they should campaign for the Marcos-Duterte ticket.”

She added that it was also “surreal” that PDP-Laban, which was set up in the early 1980s in opposition to the Marcos dictatorship, was now supporting his son.

Marcos spokesman Vic Rodriguez welcomed the endorsement as he warned of “growing rumours of possible poll fraud in the coming elections”.

The public backing of Marcos Jr comes as Robredo’s volunteer-driven campaign appears to be gathering momentum.

Crowd sizes have swelled at recent rallies.

On March 20, more than 130,000 of her supporters flocked to an event in the suburbs of Manila, standing for hours in hot and humid weather.

But the odds were stacked against her, said Eurasia Group analyst Peter Mumford.

“She enjoyed a relatively late surge in the 2016 vice-presidential race, jumping from third place and just over 20 per cent support in mid-March to win with over 30 per cent support in May – but the gap is far wider this time,” he said.